You can boil the cauliflower until just tender, saute the mushrooms one day in advance, and store them in the refrigerator separately in airtight containers. The next day, prepare the sauce and bake. Alternatively, you can blanch the cauliflowers, sautee the mushrooms, prepare the cheesy sauce, toss everything in the sauce, add to the baking dish, cover the casserole dish with cling film or a lid, and store in the refrigerator until you are ready to bake the next day. Thaw for 30 minutes at room temperature before baking.