I like to use brown mushrooms in this recipe because it makes the casserole look more presentable than white mushrooms. So I use baby Bella or crimini mushrooms. I use quite a lot of mushrooms - 32 oz. But keep in mind that mushrooms lose significant volume after they are cooked. Once cooked, they fit nicely in a 2-quart casserole dish. Since I'm lazy, I buy pre-sliced mushrooms at Whole Foods. I also buy pre-chopped onions. Which makes this casserole super easy!