Cook noodles for 10 minutes without salt. Blend soup and milk until smooth. Add mustard, half of the cheese, smoked beef, mushrooms and cooked noodles. Pour mixture into greased 1 1/2 quart casserole. Spread butter on toast and cut into small cubes. Sprinkle over casserole. Top with remaining cheese. Bake at 350° for 30 minutes.