Prep the pan: Heat the oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or oil.

Cook the sausage: In a large skillet over medium heat, brown the sausage, breaking it into crumbles. Cook until no longer pink and slightly crispy at the edges. Drain excess fat if needed.

Sauté aromatics: Add the diced onion and bell pepper to the pan. Cook 3–5 minutes until softened. Stir in garlic for the last 30 seconds. Season lightly with salt and pepper.

Layer the base: Spread the cubed bread or hash browns in the prepared baking dish. Scatter the sausage and veggie mixture evenly over the top. Sprinkle 1 1/2 cups of cheese over everything.

Make the egg mixture: In a large bowl, whisk the eggs, milk or half-and-half, 1 teaspoon salt, 1/2 teaspoon black pepper, and the dried mustard or paprika if using. Whisk until smooth and slightly frothy.

Combine: Pour the egg mixture slowly and evenly over the layered ingredients. Press down gently with a spatula so the bread absorbs the liquid.

Rest (optional but helpful): For the best texture, let the casserole sit for 10–15 minutes on the counter. If making ahead, cover and refrigerate up to 12 hours.

Bake: Sprinkle the remaining 1/2 cup cheese on top. Bake uncovered for 35–45 minutes, until the center is set and the top is golden. A knife inserted in the center should come out mostly clean with no loose liquid.

Finish and serve: Let it rest 10 minutes to set. Garnish with chopped chives or parsley. Slice into squares and serve warm.