Prepare a slow cooker by spritzing the liner with nonstick cooking spray.
To the slow cooker, add the ground beef, onion, enchilada sauce, tomatoes with green chiles, black beans, corn, garlic, and taco seasoning. Stir to combine.
1 pound ground beef, 1 small onion, 19 ounces red enchilada sauce, 10 ounces tomatoes with green chiles, 15.5 ounce black beans, 15 ounce corn, 1 teaspoon minced garlic, 2 Tablespoons taco seasoning
Cook on low for 4 to 6 hours.
Stir the tortillas and 1 cup cheese into the slow cooker. Smooth the top evenly. Top with the remaining cheese. Cover the slow cooker and continue cooking for 30 minutes.
16 corn tortillas, 2 cups shredded cheese cheddar
Before serving, top the casserole with the diced avocado, tomatoes, and cilantro.
1 avocado, 1 roma tomatoes, 3 tablespoons freshly chopped cilantro