On a foil lined cookie sheet, lay out bacon in a single layer. Place in cold oven, then turn oven on to 375º for 30 minutes, or until bacon is crispy. Meanwhile, in a large skillet, melt butter over medium-high heat. Saute` onions with mushrooms til tender. Drain off excess juice (save & freeze for "Easy Beef Stroganoff"!) In a large bowl, whip eggs together with milk & seasonings. Drain cooked bacon on paper towels & chop up. Turn oven up to 400º. Now, in greased 9 x 13 casserole dish, layer frozen hash browns, bacon pieces, then onions & mushrooms. Pour egg mixture evenly over top. Bake uncovered for 30 minutes. Pull casserole out & top with cheese. Resume baking another 15 - 20 minutes, making sure eggs are set & fully cooked through. NOTE: If you top with cheese first before partially cooking, your eggs will not set up or cook through!