Cook bacon until crisp. Remove, cover and refrigerate. Reserve 1 T. drippings. Add onion, bell pepper and mushrooms, cook 4 minutes over medium heat, 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 onion mixture evenly on top. Sprinkle with 1 cup of cheese. Spread remaining hash browns over top. Pour egg mixture on top. Cover; refrigerate overnight.
Heat oven to 325. Uncover; bake 65 minutes. Sprinkle with remaining 1 cup cheese and bacon. Bake 5 - 10 minutes longer until knife inserted in center comes out clean, and cheese is melted. Let stand 5 minutes.
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