Spray a 9X13 baking pan with a non-stick cooking spray.
Brown the sausage in a skillet over medium high heat until crumbly and browned. Drain off the excess grease.
Cut the crust off the bread and cut the bread pieces in half. Layer the bread slices in a single layer in the bottom of the baking dish. Top the bread with the browned sausage and the shredded cheese.
In a large mixing bowl, whisk together the eggs, milk, mustard, pepper and salt. Pour this mixture over the cheese in the casserole pan. Cover the dish and refrigerate for approximately 8 hours (overnight works great).
Preheat the oven to 350 degrees F. Sit the casserole dish out at room temperature while the oven preheats.
Uncover the casserole and bake for 40 minutes until the top of the casserole is golden brown. Let the casserole sit for 10 minutes and then it’s ready to serve and enjoy!
Notes
Refrigerate the leftovers in an airtight container for up to 5 days.