Uneven browning on bagels

Toast

Hi all (I am a long-time reader, first-time poster!)

I am trying to master bagel-making but am having problems with uneven browning. As you can see from the picture, the bottoms of the bagels get too dark where they touch the baking sheet, and are too pale on the sides. In fact, the bagels were slightly underdone because I didn't want them to burn where they touched the sheet.

Any advice on how I can get even browning? I'd like the color to be the midpoint of the two extremes. 

Should I use bagel boards? 

My current setup:

Flour: KA Hi-Gluten flour

Hydration: 57% 

Technique: Hamelman

Oven temp: 425F

Baking surface: aluminum sheet pan with Silpat on top

Many thanks for all the help you've all already given me over the years..

 

I tried using a bagel board (burlap stapled to a 2x4), with a baking steel positioned on a rack above it to radiate heat down. Somewhat remarkably, the opposite thing happened. The bagels browned on the sides but not the tops and bottoms.

Can anyone help me make sense of these results?