Prep your pan and oven. Heat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or a thin layer of butter to prevent sticking.

Cook the sausage and veggies. In a large skillet over medium heat, cook the sausage until browned and cooked through. Drain excess fat. Add diced onion, bell pepper, and garlic, and sauté until softened, about 3–4 minutes. Set aside to cool slightly.

Drain the hash browns well. If they’re still a bit icy, spread them on a clean towel and pat dry. Excess moisture can make the casserole soggy.

Whisk the egg mixture. In a large bowl, whisk together eggs, milk, salt, pepper, paprika, and dry mustard until smooth and well combined.

Layer the base. Spread the hash browns evenly in the baking dish. Sprinkle with half of the shredded cheese.

Add the sausage and veggies. Evenly distribute the cooked sausage mixture on top of the hash browns and cheese.

Pour and top. Pour the egg mixture over everything, tilting the dish slightly to help it settle. Top with the remaining cheese.

Bake until set and golden. Bake for 40–50 minutes, or until the center is set and a knife inserted in the middle comes out clean. The top should be lightly browned and bubbly.

Rest and garnish. Let the casserole rest for 10 minutes before slicing. Garnish with green onions or chives.

Serve. Cut into squares and serve warm. Add hot sauce, salsa, or a dollop of sour cream if you like a little extra kick.