Heat a large skillet over medium-high heat and stir in sausage. Cook and stir until sausage is crumbly, evenly browned, and no longer pink, 5 to 7 minutes. Drain and discard any excess grease.
Beat eggs, milk, salt and ground mustard in a large bowl.
Stir Cheddar cheese and sausage into the egg mixture.
Roll out crescent roll dough and place one sheet on the bottom of a large baking dish.
Pour egg mixture into baking dish; place the second sheet of crescent roll dough on top.
Cover with plastic wrap and refrigerate for 8 hours or overnight. Allow casserole to come to room temperature for 30 minutes before baking; remove plastic.
Preheat oven to 350 degrees F (175 degrees C).
Bake in the preheated oven until egg is set and a knife inserted near the center comes out clean, about 40 minutes.