What did I do wrong?

Looks like you had great oven spring which means the dough has surged up when it went into the oven and burst at the weakest point. It is really quite dramatic. Is there a gaping hole underneath? If so it may be that you haven't quite folded the dough into shape quite right and trapped a large bubble inside? I don't claim to be an expert mind you, others with greater experience will be along to help I'm sure but could we see the interior once it's cut?

I agree, just take a sharp serrated knife (like a steak knife) or a razor blade and cut fairly deeply down the middle of the loaf before putting it into the oven.

I'm not sure how comfortable you would be with baking this loaf on a baking sheet or tray instead of shaped in a pan. It's a 'lean' bread (meaning just flour, water, salt and yeast with no enrichment such as dairy, oil or sugar) and might do well just shaped into a round or 'log' shape. You could proof it (final rise) on a piece of baking parchment on a baking sheet, then score the top and bake it. See how you like it!

that was given to me on Monday.  Not to look "a gift horse in the mouth," but to be honest, the baguette was too big, heavy and the crumb tight and close with no air pockets inside.  It took 3 hours to get here from the old capital of Luang Prabang and made in a start up bakery.  Pictures show that the brick walls of the bakery are still in construction and there are no windows, doors or screens yet.  Even though the impressing scoring opened up nicely, the flavour was flat of baked flour and not enough fermentation.  Does any of that sound familiar?  It was given to me with a big heart so  I'm thinking of ways to recycle it, like shredded crumbs, dripping in butter garlic bread, roasting, croutons or something.  I'll find a way.  

The first things that pop to mind for both is to try longer fermentation, longer bake if more crust colour is desired and add steam to the oven during the initial spring.  Perhaps as others have suggested less dough in the tin or free style and scoring.  Try one idea at a time (if you can) but get steam into the oven and score the top of the loaf if a tight "skin" has developed during the proof.  Might want to start out hot (steam will lower the temp a bit) and turn the fan or oven off for the first 5 to 10 minutes, then turn it back on after some rising and expanding using a slightly lower temperature than the pre-heat.  

Happy baking!  

Oh, and what is that neat looking contraption behind the loaf with the red mugs parked under it?   :)

Mini has suggested a number of things to try, but since you didn't specify the timing or the fermentation temperatures involved I am going to suggests a couple of more ideas to think about.

1. read about making the dough window pane before you set it to bulk ferment.  That way you know you have fully developed the gluten.  It looks pretty good, but perhaps a little under developed.  I would shoot for 67-69% water to flour ratio (by weight).  The end result is quite sensitive to dough hydration but it is also sensitive to flour quality.  Your flour seems to be strong enough to do what you want it to do.

2. 7g of yeast for 500g of flour is 1.4%.  That seems high to me, and too much yeast and too little fermentation will produce the result you see.  I would suggest perhaps half that much yeast for that much flour. Check your dough temperature and perhaps adjust the bulk fermentation time to let it fully double in volume before shaping. Fermentation temperatures between 72°F and 78°F should double in about 1 hr to 1:15 but don't get anxious if it is slow, there are lots of reasons for that and unless you are in a hurry (which you should not be) just let it go.  There are lots of videos on YouTube that can help you learn about how to judge when it is ready to shape.

3. Under-proofed.  You could easily take 30% of the dough out of the pan and it would still rise well over the top before you put it into the oven.  I suspect that your pan was intended to make a smaller loaf so try scaling the formula to 600g and waiting for the dough to fully double before you put it in the oven.

Wait until the dough is soft but not flabby before baking - this won't make much sense now, but you will quickly come to understand it if you do a split batch and wait an hour between baking the two loaves.  If the second one collapses, try waiting just 30 min the next time.  You really need to run this experiment so that YOU get to know what an over-proofed loaf looks/feels like.  Flabby is when it falls if you touch it or pick up the pan.  Plan to bake 20 min before that happens :-)

The oven temperature is probably a little low - I would go with 200°C for 35 min and if it looks too dark reduce the temperature by 25°C for the last 10-15 min. Check the temperature with an oven thermometer.

one teaspoon of salt it 5g or 1% of 500g of flour    a bit low

two level teaspoons would be 10g or 2%         average   

So depending on your preference, you can add a little.  That also tends to help with crust colour and rising.

loaf yesterday and it was definitely too much dough for a standard bread pan.  (I don't have one here.)  I shaped and pinched a nice thick dough log, pinched it shut and dusted it with flour.  I put it top side down into a long banneton.  You probably don't have one of these things but you can also make a big cloth donut twisting a towel and cover it with another towel.  Dust with some flour to prevent sticking and prop up the sides with some canned food or noodle boxes, whatever.  Deep is more important the wide.

When the risen dough looks almost double in size, carefully remove the supports and roll it over onto baking parchment or floured cardboard so it slides easily into a hot oven.  I flip my baking tray upside down in the oven so I don't have to deal with the edge and that same edge helps trap heat under the baking surface.  So now you can decide to score or not score.  If the dough is very soft and starting to flatten out, don't score.  If it seems to be holding it's shape to an acceptable degree, try scoring it.  

Think of the exposed dough skin as a basket holding the dough, where you cut, the basket will open.  You want to leave enough unscored dough on the sides to hold up the loaf and score in such a way that the dough doesn't pour out sideways.  Many single side scores roll back like a roll top desk - sort of like your loaf that tore along the side.  You can try that kind of score along one side and the top will open and rotate to the other side.  

To break up the top but still leave some bands across to hold up the sides, like handles do for a basket,  Try some diagonal cuts.  Imagine a toy train track running down the middle of the loaf.  A one inch strip from one end to the other.  The trick is to stay inside those imaginary lines.   Start with a sharp edge at the end of the loaf and cut a 4 inch line from one side of the track to the other.  Back up, start half way  up from the first cut and about half an inch away and make another cut from one side of the imaginary track to the other, also about 4 inches long to the other end of the loaf.  Now slide it into the hot oven before it expands too much.  Next time try it adding an angle to the cut so you can compare them.  

My loaf took 30 minutes to bake at 200°C.  I started with 40 on the timer and 250°C (the oven cooled drastically when I opened the door)  but it was looking and smelling so good and nicely brown I took it out and knocked on the bottom listening for clean hollowness.  

Details... just for info... The basket I used during the final proof is 5 inches by 10.5 inches long on the inside and 3" deep.  I let it rise to the edge and dome up about an inch above the edge in the middle.  The finished loaf came out 12" long and averages 6" wide and just under 4" high. 

I can post a pick if you're interested.

Mini