Use bacon drippings to sauté onions and mushrooms. Add flour, mix well. Add cream and stir until it thickens. Add cheese and stir until smooth. Add remaining ingredients with the exception of the onion rings. Mix well.
In buttered 9x13x2 casserole, place the above mixture. Bake at 350° for 20 minutes.
Remove from oven and top with French Fried onions rings. Bake an additional 5 to 10 minutes.