Overnight Bacon Breakfast Casserole

Prepare a 9x13-inch baking dish by applying a generous amount of cooking oil or butter to prevent sticking. Arrange the brioche cubes in an even layer within the prepared pan.

In a skillet, cook the bacon over medium heat, stirring occasionally, until it reaches your desired level of doneness, approximately 10 minutes. Remove from heat and let the bacon cool slightly, around 5 minutes.

Heat a small amount of rendered bacon fat in the same skillet over medium-high heat. Sauté sliced onions and bell peppers until they're tender, taking about 5 to 7 minutes.

In a large mixing bowl, whisk eggs until they're well beaten. Combine milk, shredded Cheddar cheese, mustard powder, black pepper, and salt into the eggs. Once you've mixed everything together, incorporate the cooked bacon and sautéed onion-bell pepper mixture.

Carefully pour the egg mixture over the brioche bread cubes, ensuring they're evenly distributed. Gently press the bread cubes into the egg mixture as needed to ensure all the bread is submerged in the liquid. Cover and refrigerate for 8 hours or overnight to allow the mixture to set.

Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius). Place the casserole in the preheated oven on the middle rack and bake until it's set, the top is golden brown, and a toothpick inserted into the center of the casserole comes out clean, approximately 45 to 55 minutes. Remove from the oven and let it cool slightly before serving, around 5 to 10 minutes.