In a large skillet, cook sausage and mushrooms over medium heat until meat is no longer pink; drain.
Place half of the bread cubes in a greased 13-in. x 9-in. baking dish; top with 2 cups sausage mixture and half of the cheese and tomatoes. Repeat layers. In a large bowl, whisk the eggs, milk, mustard, salt and pepper; pour over bread mixture.
Bake, uncovered, at 350degrees for 50-55 minutes or until a knife inserted near the center comes out clean. Let stand for 10 minutes before serving.
Letting this casserole rest for ten full minutes after it comes out of the oven allows the coagulated egg proteins to set properly so the portions hold their shape cleanly when sliced. Building a layered strata requires a baking dish measuring precisely thirteen by nine inches to ensure the liquid custard distributes evenly across the cubed bread without pooling at the edges. Since counter space is often at a premium in smaller kitchens, assembling the components directly inside a disposable foil pan simplifies cleanup and eliminates the need to transfer heavy ceramic dishes into crowded refrigerators. Ground mustard acts as an emulsifier in the liquid dairy mixture, helping keep the fats from separating as the eggs bake at three hundred fifty degrees.
Preparing and Freezing Ahead
Assembling the dish the night before gives the bread cubes adequate time to absorb the liquid without becoming overly soggy. Tightly wrapping the unbaked pan in multiple layers of plastic wrap protects the dairy fats from absorbing unwanted odors in the freezer for up to one month. Thawing the casserole completely in the refrigerator overnight prevents the exterior from burning before the dense, frozen center reaches the safe internal temperature required for fully cooked eggs.
Baking and Equipment Workarounds
Standard ovens can exhibit hot spots that cause uneven browning across a wide casserole dish, making it wise to rotate the pan halfway through the fifty-to-fifty-five-minute baking window. Inserting a thin metal knife near the exact center provides a reliable visual cue; if the blade emerges without any wet residue clinging to the steel, the structure is set. When dealing with a toaster oven or a compact wall unit that cannot accommodate a standard pan, dividing the recipe proportionally into two smaller eight-inch square baking dishes maintains the identical ingredient ratios and cooking times.