Meanwhile, in 3-quart saucepan, melt butter over low heat. Reserve 1 tablespoon of the butter in small bowl. Stir bell pepper and onions into butter in saucepan. Increase heat to medium; cook and stir 1 minute. Stir in flour, salt, pepper and mustard. Cook, stirring constantly, until mixture is bubbly. Increase heat to medium-high. Gradually add milk, stirring constantly, until mixture boils and thickens, about 5 minutes. Gently stir in bacon and pasta.