Preheat oven to 350 degrees. Spray a 9 x 13-inch casserole with nonstick cooking spray for ease in cleaning.
Brown ground beef in a large skillet over medium heat. When the ground beef is about halfway browned, add the onion. Cook until the beef is browned and the onion is soft. Reduce heat to low. Add garlic and cook for 1 minute, stirring continuously. Drain any excess grease. Add 1 1/4 cups enchilada sauce, black beans, and green chiles; cook for 1-2 minutes while stirring to combine.
Spread 2 tablespoons of enchilada sauce on the bottom of the casserole dish. Layer 1/4 of the tortillas on the bottom, then spread 1/3 of the beef mixture, and 1/4 of each cheese. Repeat these layers two more times. Top with the last layer of tortillas, the remaining enchilada sauce, and the remaining cheese.
Cover the pan with foil and bake for 30-35 minutes or until the cheese is melted and the casserole is heated through. Cook covered for 5 minutes.
Toss avocado with lemon juice. Top the casserole with avocado, tomato, cilantro, and jalapeno.