Step 1: Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish thoroughly with butter or non-stick spray.
Step 2: In a large skillet, cook the breakfast sausage over medium heat, breaking it up with a spoon, until it's browned and cooked through. Drain off any excess grease and set aside. In the same skillet, cook the bacon until crispy. Remove the bacon, drain excess fat, and crumble once cool. Leave about a tablespoon of bacon fat in the pan.
Step 3: Add the diced onion, bell pepper, and mushrooms to the skillet with the reserved bacon fat. Sauté over medium heat for 5-7 minutes, until the vegetables are tender-crisp. Season with a pinch of salt and black pepper.
Step 4: In a large bowl, whisk together the eggs, half-and-half, Dijon mustard, salt, black pepper, garlic powder, and onion powder until well combined and slightly frothy.
Step 5: Layer the ingredients in the prepared baking dish. Spread the cubed bread evenly across the bottom. Sprinkle half of the cooked sausage and half of the sautéed vegetables over the bread. Scatter half of the shredded cheese (cheddar and Monterey Jack) over the top. Repeat with the remaining sausage, vegetables, and cheese, finishing with the remaining crumbled bacon.
Step 6: Carefully pour the egg mixture evenly over all the layers in the baking dish. Gently press down on the layers with a spatula to ensure the bread soaks up the egg mixture.
Step 7: Cover the baking dish tightly with aluminum foil. Refrigerate for at least 30 minutes, or preferably overnight, to allow the flavors to meld and the bread to fully absorb the egg mixture.
Step 8: When ready to bake, remove the casserole from the refrigerator and let it sit at room temperature for about 15-20 minutes while your oven preheats to 375°F (190°C). Bake, covered, for 30 minutes.
Step 9: Remove the foil and continue baking for another 20-30 minutes, or until the casserole is set in the center, golden brown on top, and the cheese is bubbly. An internal temperature of 160°F (71°C) indicates doneness. If the top browns too quickly, you can loosely tent it with foil again.
Step 10: Let the casserole rest for 10-15 minutes before slicing and serving. Serve warm with your favorite breakfast sides.