going to attempt Yotam Ottolenghi’s Cauliflower Cake (https://www.thekitchn.com...). am intolerant to onions and NEED a good substitute (don't think my usual carrots and celery will cut it)!
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.