Preheat a large skillet over medium-high heat.

Add the sausage to the skillet.

Cook, stirring, until the sausage is crumbly and browned (5-7 minutes).

Drain off any excess grease.

In a large bowl, beat together the eggs, milk, salt, and ground mustard.

Stir in the shredded Cheddar cheese and cooked sausage.

Unroll one sheet of crescent roll dough and place it on the bottom of a large baking dish.

Pour the egg mixture into the baking dish.

Place the second sheet of crescent roll dough on top.

Cover the baking dish with plastic wrap and refrigerate for at least 8 hours, or overnight.

Remove the plastic wrap and let the casserole come to room temperature for 30 minutes before baking.

Preheat the oven to 350 degrees F (175 degrees C).

Bake the casserole in the preheated oven for about 40 minutes, or until the egg is set and a knife inserted near the center comes out clean.