Spray a 9 x 13-inch baking dish with cooking spray.
Evenly scatter bread cubes across baking dish; sprinkle cheese over bread cubes.
In a mixing bowl, whisk together eggs, milk and mustard; pour evenly over the cheese and bread.
Sprinkle minced onion and freshly ground pepper over mixture.
Evenly sprinkle bacon and sausage crumbles over the casserole; gently push into the egg mixture.
Next, even sprinkle mushroom and tomatoes over top; again, gently push into the egg mixture.
Cover casserole dish and refrigerate overnight or 6-8 hours.
To prepare: Place cold dish in oven, then turn oven to 325 degrees. This allows the dish to warm up with the oven and avoid breakage. Bake casserole 45-50 minutes, until "set" in center. Cover with foil after about 15-20 minutes of baking, so as to prevent casserole top from browning too quickly.