Preheat oven to 375 degrees. Grease a 9 x 13 inch baking dish with butter. Set aside.
In a large sauté pan over medium heat, add the breakfast sausage. Sauté the sausage and break it up into little pieces as it cooks. Continue cooking for 8 to 10 minutes. The sausage should be browned and crispy in spots and cooked through. Stir in the spinach and sauté until wilted. Remove from pan to a bowl and set aside.
In a large bowl, add the eggs and whisk until combined. Whisk in the milk, mustard, baking powder, salt and pepper until thoroughly combined.
Stir in the cooked sausage, spinach and cheddar cheese.
Pour the mixture into the prepared baking dish.
Bake for 25 to 30 minutes or until the center is just set.
Serve warm or at room temperature.
Notes
Adapt the recipe to include your favorite breakfast casserole ingredients. Some delicious options include cooked and crumbled bacon, diced ham, Swiss, pepper jack or goat cheese, steamed broccoli or asparagus, sauteed mushrooms or finely diced tomatoes (remove the seeds).