Combine 1/2 cup cheese, 1/4 cup sour cream, 1/2 cup onion, 1/4 teaspoon salt, 1/2 teaspoon black pepper, and 1 egg in a large bowl, stirring to combine. Stir in hash browns. Heat 2 tablespoons oil in a large nonstick skillet over medium heat; spread hash brown mixture into pan, and cook 7 minutes or until browned on one side. Drizzle hash brown mixture with remaining 2 tablespoons oil; flip with a spatula, and cook 7 more minutes or until browned on the second side. Spoon evenly into the bottom of a lightly greased 3-quart baking dish. Add pancetta to skillet over medium-high heat; cook, stirring frequently, 2 minutes. Add red bell pepper and remaining 1 cup onion; cook, stirring occasionally, 2 minutes. Whisk together milk, 5 eggs, remaining 1/2 cup cheese, remaining 1/2 cup sour cream, remaining 1/2 teaspoon salt, and remaining 1/2 teaspoon black pepper in a medium bowl. Stir pancetta mixture into egg mixture, and pour over hash browns. Cover with aluminum foil, and refrigerate 8 hours or overnight. Preheat oven to 350°. Bake, covered, 50 minutes or until top is browned and eggs are set. Garnish, if desired.