Preheat oven 350 ºF (180 ºC).

Cook pasta according to package directions. Drain and set aside.

Meanwhile, cook beef, onion and green pepper in a large frypan over medium-high heat until meat is browned; drain off fat. Stir in tomato sauce, basil and oregano. Simmer 3 minutes. Spoon into 8 cup (2 L) shallow rectangular baking dish; set aside.

Melt 4 tbsp (60 mL) of the butter in a medium saucepan. Blend in flour and mustard. Gradually stir in milk. Cook over medium heat, stirring constantly, until mixture boils and thickens. Remove from heat. Add cheese and stir until melted.

Stir in cooked pasta. Spoon pasta and cheese over meat layer in baking dish.

Melt remaining butter (2 tbsp / 30 mL); combine with breadcrumbs. Sprinkle over pasta and cheese layer.

Bake in oven 15 to 20 minutes or until hot and bubbly.