Heat oven to 350°. Sauté celery, pepper, onion and mushrooms in butter for 5 minutes. Blanch asparagus in boiling water for 1 to 2 minutes or until bright green. Drain. In a large buttered baking dish, mix all ingredients except crumb topping. Bake for 35 to 40 minutes. Crush Ritz crackers in a plastic bag. Transfer to bowl and mix with 1 tablespoon butter. Add to top of casserole and return to oven for about 5 minutes. Yields 8 to 10 servings.