Preheat oven to 400. Meanwhile spray a shallow baking dish with nonstick coating. Place fillets in dish, tucking under any thin edges.
Melt margarine or butter; brush on top of fish. Top with bread crumbs. Bake for 10 to 15 minutes or until fish flakes with a fork.
While fish is baking, prepare sauce. In a small saucepan stir together milk and flour. Cook and stir over medium heat till thickened and bubbly. Add cheese and mustard. Cook and stir until cheese melts.
Serve fillets with cheese sauce and dillweed (optional).