Dried mushrooms and dried mushroom powder. Do not snort.
At some point (soon, I hope, but y'all know I'm a liar) I'll teach you how to make a quick southern-style casserole out of things you already have in your kitchen. Won't that be fun? But first things first. What do you do when you don't have the "1 can cream of mushroom soup" your mother's recipe calls for?