1. In a large pot of boiling water, slowly add the macaroni. Boil for 10 minutes, stirring occasionally.

2. Drain and place in a buttered 9×13 inch baking dish (or use cooking spray or margarine).

3. Melt the butter in a large saucepan over low heat. Remove from heat and whisk in flour and mustard until smooth. Return to medium heat. Gradually add milk, stirring constantly, until the sauce thickens and is smooth. Bring to a boil.

4. Gently add the cheese pieces to the boiling milk mixture, stirring until all the cheese has melted.

5. Pour the cheese mixture over the macaroni, stirring as you go. Make sure the cheese covers all the macaroni.

6. Bake in the oven at 350°F for 45 minutes, or until golden brown and bubbly.
Slow cooker: butter the slow cooker, add the macaroni and cheese mixture. Mix well. Set to medium power and cook for 7 hours.