Preheat oven. Preheat oven to 350. Grease a 9x13 casserole dish with butter.
Cook the sausage. Heat oil in a large cast iron skillet. Add sausage and cook until browned, breaking up as it cooks. Remove from heat and drain excess liquid. Set aside.
Mix eggs, milk, and seasonings. In a large bowl, add in eggs, milk, ground mustard, salt, and pepper. Whisk until fully combined.
Combine ingredients. Add eggs to casserole dish. Stir in sausage, sourdough bread, and cheese. Pour into greased baking dish.
Bake. Bake the casserole for about 1 hour or until the center is set. Broil 1-2 minutes to get a crisp top. Remove from oven and allow to cool for about 10-15 minutes.
Serve & enjoy! Cut the breakfast casserole into slices, serve and enjoy!
Notes
Store in an airtight container in the fridge for up to 4 days.
For best results, serve warm.
After baking, broil for a few minutes for the most delicious crisp top and edges.