I'd like to make a gluten-free version of a fish casserole recipe. The recipe calls for making "4 1/2 cups of thin cream sauce, made with heavy cream, fish water, flour & butter" which is placed in a casserole with seafood and boiled rice, covered with a little grated cheese, and baked. Is there anything I could successfully substitute for the flour? If not, is there another way to go about it entirely? Thanks for your help.