Brown the ground beef in a large skillet over medium heat. Add the diced onion and minced garlic, then cook until the onion softens.
Stir in the black beans, enchilada sauce, chili powder, cumin, and salt. Let the mixture simmer for 5 minutes so the flavors combine.
Spread a thin layer of the beef mixture in the bottom of a greased 9x13 baking dish. Place two tortillas over the mixture, tearing them if needed to fit.
Spoon half the remaining beef mixture over the tortillas and sprinkle with 1 cup of cheddar cheese. Repeat with the last two tortillas, the rest of the beef mixture, and the remaining cheese.
Bake at 375°F for 20 minutes until the cheese melts and the edges turn golden. Remove from the oven and top with chopped green onions before serving.