Butter your 9x13 inch baking dish generously.
This prevents sticking later.
Set it aside for now.
Cut bread into 1-inch cubes.
Spread them evenly in the dish.
Use slightly stale bread for best results.
Sprinkle cooked sausage over the bread.
Add both cheeses on top.
Spread everything evenly throughout.
Whisk eggs in a large bowl.
Add milk, mustard, salt, and pepper.
Mix until completely smooth and combined.
Pour egg mixture over the bread slowly.
Press bread down gently with a spoon.
Make sure everything gets coated well.
Cover dish with plastic wrap tightly.
Refrigerate for at least 8 hours.
This soaking time is very important.
Remove from fridge 30 minutes early.
Preheat oven to 350°F.
Drizzle melted butter on top.
Bake for 45-50 minutes uncovered.
The top should be golden brown.