Preheat oven to 400 degrees.
Boil water and cook pasta shells as instructed on box. When there are 3 minutes left, add your cauliflower to the boiling pot. Drain and set aside.
In a large saucepan melt your butter over medium heat. Slowly add flour, dry mustard and pepper until smooth.
Add your milk and cheese slowly and keep stirring until it's smooth and melted. Take off burner.
Add your macaroni and cauliflower to cheese mixture saucepan and stir until all shells are coated.
Transfer your mixture to a baking dish and sprinkle bread crumbs on top.
Cover and bake for 25-30 minutes or until cheese is bubbly.