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.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.