Heat a large skillet over medium-high heat and stir in sausage. Cook and stir until sausage is crumbly, evenly browned, and no longer pine, about 10 minutes. Drain and discard any excess grease.

Beat eggs in a large bowl; stir in milk, salt, and mustard, Fold bread cubes, cheddar cheese, and sausage into egg mixture. Pour into a lightly greased 7×11 baking dish. Cover and refrigerate for 8 hours or overnight.

Preheat oven to 350 degrees F. Remove casserole from the refrigerator 30 minutes before baking.

Bake in preheated oven until a knif inserted near the center comes out clean, about 40 minutes.