Cook macaroni until tender in boiling water. Drain and set aside.
In a 2 quart saucepan, melt butter, stir in flour, mustard and salt. Then add milk, and stir constantly over medium heat until mixture boils and thickens slightly (mixture will be thin). Turn burner to low, add shredded cheese and stir until melted. Remove from heat .
Sitr in noodles, put in a buttered 2 quart casserole dish and bake about 25-30 minutes in 350° oven.