Step 1

Melt the butter in a skillet over medium heat, and cook and stir the mushrooms until their liquid has mostly evaporated and the mushrooms are beginning to brown, about 10 minutes. Set the mushrooms aside.

Step 2

Grease a 9x13 inch baking dish, and arrange 8 muffin halves, cut sides up, in the bottom of the dish. Stuff pieces of remaining muffins into the spaces between the halves. Spread 1 cup of Colby-Monterey Jack cheese over the muffins in a layer, and distribute the asparagus pieces, mushrooms, red pepper, and onion over the cheese.

Step 3

Whisk together eggs, milk, salt, dry mustard, and black pepper in a bowl, and pour the egg mixture over the muffins and vegetables. Cover the dish, and refrigerate overnight.

Step 4

The next day, preheat oven to 375 degrees F (190 degrees C).

Step 5

Remove the casserole from the refrigerator and let stand for 30 minutes. Spread 1 cup of Colby-Monterey Jack cheese in a layer over the casserole.

Step 6

Bake in the preheated oven until a knife inserted near the edge comes out clean, 40 to 45 minutes. Let stand 5 to 10 minutes before cutting into squares.