- In a large skillet, brown the mushrooms and onion in the butter. Season with salt and pepper. Transfer to a plate.
- In the same skillet, brown the sausage meat, breaking it with a wooden spoon. Add butter, if needed. Transfer to the plate with the mushrooms.
- On a work surface, cut the rounded top of the bread. Remove some of the crumb to form a cavity, leaving about 1-inch (2.5 cm) thick of crumb and crust around. Place in a baking sheet lined with parchment paper. Set aside.
- Cut 2 cups (500 ml) of the removed crumb into cubes. Set aside. Keep the remaining crumb for another use.
- In a large bowl, whisk together the eggs, milk, cheese and mustard. Add the vegetables, sausage and bread cubes and mix thoroughly. Season with salt and pepper.
- Pour into the bread cavity. Cover with plastic wrap and refrigerate overnight.
- With the rack in the middle position, preheat the oven to 350 °F (180 °C).
- Bake for about 1 hour or until the eggs are cooked. Let rest for 10 minutes. Cut into slices and serve.