Add the eggs to a very large bowl, and whisk them until all of the yolks are broken.
Pour in the milk, add the ground dry mustard and salt, and whisk to combine.
Add in the cubed sourdough bread, as well as the cooked and diced sausage (or bacon). Stir well to combine and coat the bread with the egg and milk mixture.
Add in the shredded cheese, and fold it in until well combined.
Grease a 9X13 baking dish with butter or avocado oil, and pour the breakfast casserole mixture into the dish.
Cover the casserole and place it in the fridge for at least 2 hours, or up to overnight.
When you’re ready to bake, place the dish in a cold oven, and then heat it to 350ºF.
Once the oven has preheated (with the dish inside), start the cook time. Bake for 35-45 minutes, or until the center is set.
Bake covered with foil or a lid if you like a soft texture, or uncovered if you like crispy edges and a crispy top.
Once the egg is set in the center, remove from the oven and allow to cool for 5 minutes before slicing and serving.