• Preheat Oven: Set your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or butter.
• Cook Sausage: In a skillet over medium heat, cook the breakfast sausage until browned and no longer pink, about 5-7 minutes. Break it into crumbles as it cooks. Once done, drain any excess grease.
• Sauté Vegetables (Optional): If using bell peppers and green onions, add them to the skillet and sauté for 2-3 minutes until softened.
• Prepare Hash Browns: In the same skillet, add the thawed hash browns. Cook over medium heat, stirring occasionally, until they begin to lightly brown, approximately 8 minutes.

Assemble Casserole:

• Spread the cooked hash browns evenly in the prepared baking dish.
• Layer the cooked sausage (and vegetables, if using) over the hash browns.
• Sprinkle 1.5 cups of shredded cheddar cheese over the sausage layer.
• Mix Egg Mixture: In a large bowl, whisk together the eggs, whole milk, salt, black pepper, and onion powder until well combined.
• Combine and Chill: Pour the egg mixture over the layers in the baking dish, ensuring it covers evenly. For best results, cover the dish with plastic wrap or foil and refrigerate for at least 8 hours or overnight. This allows the flavors to meld and the casserole to set.

Bake:

• If refrigerated overnight, remove the casserole from the fridge and let it sit at room temperature for about 10 minutes.
• Preheat the oven to 350°F (175°C) if not already done.
• Bake the casserole, uncovered, for 45 minutes, or until the eggs are set and the top is golden brown.
• Add Remaining Cheese: Sprinkle the remaining 0.5 cup of shredded cheddar cheese over the top of the casserole. Return it to the oven for an additional 5 minutes, or until the cheese is melted and bubbly.
• Serve: Once done, remove the casserole from the oven and let it rest for 5 minutes before slicing and serving.