Preheat the oven to 190 C / 375 F. Grease a 9x13-inch (23x33 cm) baking dish with butter or oil.

Heat a 12-inch skillet over medium heat. Add the bulk pork breakfast sausage and cook, breaking it into crumbles, for 6 to 7 minutes until browned and no longer pink. Drain off excess fat and transfer the sausage to a large mixing bowl.

In the same skillet, melt the butter over medium heat. Add the onion and cook for 3 minutes until soft. Stir in the black beans and green chiles and warm through for 2 minutes, then add to the bowl with the sausage.

Whisk the eggs with the salt, pepper, and cumin. Pour into the skillet and cook over medium-low heat, stirring often, for 3 to 4 minutes until just set but still a little wet, they'll finish cooking in the oven. Fold the eggs into the sausage and bean mixture along with half the shredded cheddar or Mexican blend cheese.

Wrap the tortillas in a damp kitchen towel and microwave for 30 seconds so they bend without cracking.

Spread 1/2 cup of the enchilada sauce over the bottom of the prepared baking dish.

Lay a tortilla flat, spoon about 1/3 cup of the egg filling down the center, and roll it up snugly. Place seam-side down in the dish. Repeat with the remaining tortillas, packing them in tight rows.

Pour the remaining enchilada sauce over the top and scatter the rest of the shredded cheddar or Mexican blend cheese evenly.

Bake for 20 to 22 minutes, until the cheese is melted and the sauce bubbles at the edges. An instant-read thermometer inserted in the center should read at least 165 F / 74 C.

Let the casserole rest for 5 minutes. Top with sour cream, cilantro, green onion, and avocado before serving.