Prep the dish and oven: Grease a 9x13-inch baking dish with butter or oil. Preheat the oven to 350°F (175°C) if baking right away.

Cook the sausage: In a large skillet over medium heat, crumble and cook the sausage until browned and cooked through, about 6–8 minutes. Drain excess fat if needed, leaving about a tablespoon for flavor.

Sauté aromatics: Add the onion and bell pepper to the skillet. Cook until softened, 4–5 minutes. Stir in the garlic and cook 30 seconds more, just until fragrant. Remove from heat.

Toast the bread (optional but helpful): If your bread is very soft, lightly toast the cubes in the oven for 8–10 minutes to dry them out. This helps the casserole hold its shape and prevents sogginess.

Whisk the egg mixture: In a large bowl, whisk eggs, milk, salt, pepper, smoked paprika, and dried mustard until fully combined. The mixture should be smooth and well-seasoned.

Assemble the layers: Spread the bread cubes in the prepared baking dish. Scatter the sausage and sautéed veggies evenly over the bread. Sprinkle about two-thirds of the cheese on top.

Add the custard: Pour the egg mixture slowly and evenly over everything. Gently press down with a spatula so the bread absorbs the liquid. Top with the remaining cheese.

Rest or bake: For best results, cover and refrigerate for at least 30 minutes or overnight. If you’re short on time, you can bake immediately—just note the texture will be slightly lighter.

Bake: Uncover and bake at 350°F (175°C) for 40–50 minutes, until the center is set and the top is golden. A knife inserted near the center should come out clean, and the internal temperature should reach 160–165°F (71–74°C).

Finish and serve: Let it rest 10 minutes before slicing. Garnish with parsley or chives. Serve warm with fruit, a green salad, or hot sauce on the side.