Substitute for onion in Yotam Ottolenghi’s Cauliflower Cake?

Two ideas. See what you think,, and use all of one or half of each by volume.
Sweet pepper - any color, but red or orange, with thin rings cut horizontally from the veg, will give you nice rings to contrast to the filling & replace the red onion rings.
Fennel bulb - cut in arcs or half moons, will give you about the same weight & crunch as onion, but a very different (good) flavor. Sadly, I know of no version to give replacements for the colored rings.