Preheat oven to 350 degrees. F. Lightly-butter a 1 1/2-quart baking dish.
Wash the cauliflower and remove the tough outer leaves of the cauliflower and discard. Break cauliflower into small florets.
Boil or steam cauliflower florets in water until just tender, about 4 to 6 minutes. Remove from heat and drain. Transfer the cooked cauliflower to the prepared baking dish; set aside.
In a large frying pan over medium-high heat, melt the 1/4 cup butter and saute mushrooms and celery until tender, approximately 4 to 5 minutes. Blend in flour, salt, and mustard. Gradually stir in milk and cook until mixture comes just to a boil, stirring constantly. Add cheese and stir until melted. Remove from heat.
Pour the cheese sauce over the top of the cooked cauliflower; set aside.
In a bowl, combine cracker or bread crumbs with the 2 tablespoons melted butter; sprinkle mixture over the top of the casserole.
Bake approximately 25 to 30 minutes or until the top is golden brown. Remove from oven and serve hot.
Makes 4 servings.