Preheat the oven to 350°F (175°C). Grease a 9x13-inch or similar baking dish.

Tear the bread into bite-sized pieces and spread evenly in the prepared dish.

In a skillet over medium heat, add a splash of oil. Sauté diced onion and pepper until softened, about 4–6 minutes. Add garlic and cook for 30 seconds. If using raw sausage, cook thoroughly and break up; if using pre-cooked ham or bacon, briefly warm it through.

Scatter the cooked vegetables and meat over the bread. Sprinkle most of the shredded cheese on top, reserving a little for finishing.

In a large bowl, whisk together eggs, milk, mustard, salt, and pepper until smooth.

Pour the egg mixture evenly over the bread. Press gently so the bread soaks the custard. Sprinkle the remaining cheese on top and dot with melted butter.

Let the dish rest for at least 20–30 minutes at room temperature so the custard absorbs. For deeper flavor and easier mornings, cover and refrigerate overnight (up to 24 hours).

Bake uncovered for 35–45 minutes until the top is golden and a knife inserted in the center comes out mostly clean. If the top browns too fast, tent loosely with foil.

Let the casserole rest for 10 minutes before slicing. Garnish with fresh herbs if desired and serve warm.