In a large skillet, melt butter over medium-high heat and sautee mushrooms for 5-6 mushrooms or until tender. Add the onions and asparagus and continue sauteeing for another 4-5 minutes until slightly tender.
While vegetables are cooking, lightly spray a 9x13 baking dish with cooking spray. Place the cubed English muffins in an even layer in the dish. Sprinkle 1 cup of the cheese over the muffins.
Once the vegetables are done, distribute them evenly over the English muffins.
In a medium bowl, add the eggs, milk, mustard, salt, pepper, and hot sauce if desired. Mix well. Pour over vegetables. Cover the dish and refrigerate overnight.
In the morning, preheat the oven to 375 degrees. Remove dish from the refrigerator and allow to sit at room temperature while oven is preheating. Sprinkle with remaining 1 cup of cheese.
Bake for 40-50 minutes or until center is done. Cover with foil while cooking if top begins to brown too much.
Remove from oven and let cool slightly before serving.