3 (10 oz.) pkg. frozen chopped spinach, cooked & drained
1 lb. mushrooms
1 tbsp. onion, chopped
2 tbsp. butter
2 tbsp. flour
1 tsp. salt
1/2 tsp. dry mustard
1 c. milk
1 c. cheese, shredded
3 tomatoes, sliced

Place cooked, drained spinach in large casserole dish. Saute onions and mushrooms; mix with spinach. Make cheese sauce with all remaining ingredients, except tomatoes. Pour cheese sauce over spinach mixture and top with more grated cheese. Bake in 350 degree oven for 30 minutes. Arrange tomato slices on top of spinach and heat for 10 more minutes.