Brown sausage in a skillet over medium heat.

Drain off any excess grease from the sausage.

In a large bowl, beat together eggs, milk, and mustard until well combined.

Stir in the browned sausage and grated cheese.

Cut bread slices into approximately 1-inch cubes.

Pour the sausage and cheese mixture over the bread cubes in a large bowl.

Gently mix the bread cubes to ensure they are evenly coated with the egg mixture.

Pour the mixture into a greased 9 x 13-inch baking pan.

Cover the pan with plastic wrap and refrigerate overnight (at least 8 hours).

Preheat oven to 325°F (160°C).

Remove the plastic wrap from the baking pan.

Bake in the preheated oven for 1 hour, or until the souffle is golden brown and set.

Let the souffle cool for a few minutes before serving.