Cook bacon in large skillet over medium-high heat until crisp; drain.
Carefully pour off all but about 2 tablespoons grease from skillet. Sauté bell pepper, onion, mushrooms, and garlic over medium-high heat until tender, about 5 minutes.
Spread half the bag of hash browns in bottom of greased 13x9x2-inch baking pan. Top with sautéed vegetables, half the Swiss cheese, remaining half of the hash browns, cooked bacon, and remaining cheese.
In large bowl, whisk together remaining ingredients; pour over casserole. Cover and place in refrigerator overnight.
Remove casserole from refrigerator. Heat oven to 350oF. Bake, uncovered, 45 to 50 minutes until knife inserted in center comes out clean and top is browned.