Bring a large pot of water to a boil; slowly add macaroni. Boil 10 minutes, stirring occasionally. Drain; place macaroni in a buttered (cooking spray or margarine will work) 4-quart casserole dish.
Melt butter in large pan on low heat. Remove pan from heat; blend in flour and mustard until smooth. Return to heat (medium). Gradually add milk, stirring constantly, until sauce thickens and is smooth. Bring to a boil. Carefully add cheese pieces to boiling milk mixture, stirring until all cheese pieces have melted. Pour cheese mixture over macaroni, mixing as you add. Make sure the cheese covers all the macaroni. Bake at 350 degrees for 45 minutes or until golden brown and bubbly.
For crock pot: Butter pot, add macaroni and cheese mixture. Stir to fully blend. Set pot on medium for 7 hours.