In 12-inch skillet, cook bacon until crisp. Using slotted spoon. Remove from pan to small bowl. Cover and refrigerate. Drain drippings, reserving 1 tablespoon in pan. 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 13×9-inch (3-quart) baking dish with cooking spray. Spread half of hash browns in baking dish. Spread onion mixture evenly on top. Sprinkle with 1 cup of the cheese. Spread remaining hash browns over top. Pour egg mixture on top. Cover; refrigerate 8 hours or overnight.
Heat oven to 325°F. Uncover; bake 50 to 60 minutes or until thermometer inserted in center reads 160°F. Sprinkle with remaining 1 cup cheese and the bacon. Bake 3 to 5 minutes longer or until knife inserted in center comes out clean, top is puffed and cheese is melted. Let stand 5 minutes.