Cook bacon until crisp and set aside, reserving 2 T. drippings. Add onion, bell pepper and mushrooms, cook 5-6 minutes over medium heat (in bacon drippings), stirring occasionally. Stir in mustard, salt and pepper.
In large bowl, beat milk and eggs with wire whisk.
Spray 13x9 inch baking dish with cooking spray. Spread half of hash browns in baking dish. Spread mushroom mixture evenly on top, along with 1/2 of the bacon pieces and Rotel. Mix into the potatoes a bit. Sprinkle with 1 1/2 cups of cheese. Spread remaining hash browns over top. Top with remaining bacon pieces (mixing into potatoes a bit) and pour egg mixture on top. Top with remainder of cheese.
Bake 45 minutes (uncovered) or until a knife inserted in center comes out clean, and cheese is melted.