Some people don't like mushrooms. Here were my thoughts on substitutes on Facebook in reply to someone: For homemade mushroom soups - just blend the mushrooms in in a blender or simply leave them out and rely on seasonings. How about sliced zucchini or broccoli or I read somewhere that some people use tofu chunks. Sounds weird though, hey? Cooked sliced Japanese eggplant is another idea, but if he doesn't like mushrooms, he might not like eggplant either. Or make a different casserole with sauteed sliced peppers (all different colors) and onion - just make sure they are cooked soft enough. Even celery cooked soft with onions would be good.
I was so chuffed when a lady on Facebook said this about the casserole: "I cannot say enough about this recipe! Wow! It is hands down the best low carb dish that I have ever made! And I have had some great dishes.... Thank you!!"