Spread thawed hash browns in a deep 11x14 pan; season generously with salt and pepper. Sprinkle half of the cheese over the potato layer. In a large skillet, lightly brown the Canadian bacon slices in 2 tblsp of butter; set aside. In the same skillet saute the onions, peppers, and mushrooms until tender. Spoon vegetable mixture over the cheese layer. Top with sliced tomato if using. Arrange slices of Canadian bacon over the vegetables, overlapping if necessary. Top with remaining cheese. In a medium bowl, whisk eggs with 1/2 tsp of salt, 1/4 tsp pepper, and the milk; pour over the casserole. Using a spatula, gently press all over to soak with the egg mixture. Let stand for 15 minutes. Preheat oven to 350 degrees. Bake casserole for 35 to 45 minutes, until set. Cool slightly before slicing.
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