Prep the dish. Heat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter, making sure to coat the corners and sides.

Toast the bread (optional but helpful). If your bread isn’t day-old, spread the cubes on a sheet pan and toast in the oven for 8–10 minutes. You want them a little dry to better soak up the custard.

Brown the sausage. In a large skillet over medium heat, cook the sausage, breaking it into crumbles. Cook until browned with crisp edges, about 6–8 minutes. Transfer to a plate, leaving a tablespoon of drippings in the pan.

Sauté the veggies. Add onion and bell pepper to the skillet. Cook until softened, 4–5 minutes. Stir in the garlic for the last 30 seconds. Season lightly with salt and pepper.

Whisk the custard. In a large bowl, whisk eggs, milk or half-and-half, paprika, a pinch of red pepper flakes if using, 1 teaspoon salt, and 1/2 teaspoon black pepper until smooth.

Assemble the layers. Add the toasted bread to the greased dish. Scatter the cooked sausage and sautéed veggies over the bread. Sprinkle 1 1/2 cups cheddar and 1/2 cup Monterey Jack evenly on top.

Pour and settle. Pour the egg mixture evenly over everything. Gently press down with the back of a spoon to help the bread soak it up. Let it stand for 10 minutes on the counter. This rest helps prevent dry pockets.

Bake covered, then uncovered. Cover the dish with foil and bake for 20 minutes. Remove the foil, top with the remaining cheese, and bake 15–20 minutes more, until puffed, golden, and set in the center. A knife inserted in the middle should come out mostly clean.

Finish and rest. Let the bake rest for 10 minutes so it slices cleanly. Scatter green onions or chives over the top. Serve warm.