Coat a 13 x 9 baking dish with non-stick spray and set aside.
In a large skillet, brown sausage over medium-high heat. Drain grease and set aside.
Butter one side of each piece of bread and cut into cubes. Layer half of the bread on the bottom of the prepare baking dish, then layer half of the sausage mixture, then half of the cheese. Repeat with all ingredients. Top with diced peppers.
In a medium bowl, beat eggs, milk, and mustard until blended. Pour evenly over casserole.
Cover and let it sit overnight in the refrigerator.
When you are ready to bake, remove from refrigerator and allow it to sit for 5-10 minutes. Bake at 350 for 1 hour, or until bread is golden and eggs are set.