For those who do not eat mushrooms (the garlic avoidance I never heard of before, and this IS a non-gebrokts, gluten-friendly recipe), you can easily substitute broccoli, maybe tomato, squash or make it with additional onions. I would even consider trying to do a scallopped potato variation, layering thin slices of raw potatoes with the onion batter. I never tried this, but I always think of these ideas when I have a basic recipe for something else.
And boy, is it EVER a yummy kugel. I wish I had a nickel for every person, men included (who supposedly don’t eat quiche, crustless or otherwise) who asked for this recipe. It is the one item at my table that NEVER has a scrap left over.