Though microwave cooking may not be chef-endorsed, it does help cut down on time like in this fast and simple casserole recipe.
In a small microwavable bowl, cook onion and water on high for 2 minutes. Drain and add bacon and cheeses. Toss to mix.
Layer stuffing along bottom of a 9-by-11-inch baking dish that has been prepared with nonstick cooking spray. Top with cheese mixture.
In a medium bowl, beat eggs, milk, mustard, and onion salt and pour over cheese mixture. Cover and refrigerate for several hours or overnight.
Preheat oven to 350 degrees.
Remove from refrigerator; uncover and let stand while oven heats. Bake in center of oven until puffed, golden, and knife inserted near center comes out clean, 45–50 minutes.