Asparagus Mushroom Breakfast Strata

Heat a skillet over medium heat and melt butter, stirring occasionally until it reaches your desired consistency. Add sliced mushrooms to the skillet and cook, stirring frequently, as they begin to develop a rich brown color and most of their moisture evaporates, taking around 10 minutes.

Preheat your oven to the desired temperature and grease a 9x13-inch baking dish with cooking spray. Arrange eight muffin halves, cut-side up, in the bottom of the baking dish, filling any gaps with torn pieces of muffin. Sprinkle half of the cheese over the top, followed by asparagus spears, sautéed mushrooms, red pepper slices, and chopped onion.

In a separate bowl, whisk together milk, eggs, salt, mustard powder, and black pepper until well combined. Pour this mixture over the muffin halves and vegetables, making sure everything is fully coated. Cover the dish with plastic wrap or aluminum foil and refrigerate for 8 hours or overnight.

The following day, remove the casserole from the refrigerator and let it sit at room temperature for 30 minutes. Spread the remaining cheese evenly over the top of the casserole.

With your oven preheated to 375 degrees F (190 degrees C), place the casserole in the oven and bake until a knife inserted near the edge comes out clean, taking 40 to 45 minutes. Remove from the oven and let it stand for 5 to 10 minutes before cutting into squares