Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish with butter or cooking spray.
Step 2: In a large skillet over medium-high heat, cook the ground sausage, breaking it apart with a spoon, until it's fully browned and no pink remains. If using, add diced onion, bell pepper, or jalapeños with the sausage and cook until softened. Drain off any excess grease.
Step 3: In a large bowl, whisk together the eggs, milk, salt, and black pepper until well combined.
Step 4: Pour the egg mixture into the same skillet (or a clean one) over medium heat. Scramble the eggs until they are mostly set but still slightly moist. Remove from heat immediately.
Step 5: Line the bottom of the prepared baking dish with 4-5 flour tortillas, overlapping them slightly to cover the entire surface. If necessary, tear pieces of tortillas to fill any gaps.
Step 6: Spread half of the scrambled eggs evenly over the tortillas. Then, sprinkle half of the cooked sausage over the eggs. Follow with half of the shredded cheese. Repeat these layers once more: remaining tortillas, remaining eggs, remaining sausage, and finally, the remaining shredded cheese.
Step 7: If preparing ahead, cover the dish tightly with plastic wrap or aluminum foil and refrigerate for up to 24 hours. When ready to bake, remove from the refrigerator and let it sit at room temperature for 20-30 minutes while the oven preheats.
Step 8: Cover the baking dish loosely with aluminum foil and bake for 30 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and lightly golden, and the center is set.
Step 9: Let the casserole rest for 5-10 minutes before slicing into squares and serving. Garnish with your favorite toppings like salsa, sour cream, avocado, and fresh cilantro. Enjoy your delicious Breakfast Burrito Casserole!