Cheesy Mushroom Sausage Breakfast Casserole

cooking spray
1/2 onion, chopped
8 ounces mushrooms, sliced
1 pound sausage
2 cups tater tots
6 slices bread, cubed
3 cups Cheddar cheese, shredded
1 cup heavy cream
1-1/2 cups of milk
8 eggs
1/2 teaspoon salt
1/2 teaspoon pepper
1 teaspoon Dijon mustard
1 tomato, sliced

Brown the onion, mushrooms and sausage in a large skillet until the sausage is fully cooked. 
Use the cooking spray to grease a 9x13 inch baking dish.
In the greased baking dish, dump the tater tots and evenly spread on the bottom of pan and then add a layer of cubed bread. Now add the onion, mushrooms and sausage mixture over the bread and sprinkle a layer of cheese on top.
Whisk the heavy cream, milk, eggs, salt, pepper and mustard in a medium bowl. Once whisked together, pour over the casserole and then layer with tomatoes. Cover with plastic wrap and place in refrigerator overnight.
To prepare: Preheat oven to 350 degrees.
Once the oven has been preheated, uncover the casserole and bake for 60 minutes.