Brown the ground beef and onion in a skillet over medium-high heat until no longer pink, about 5 minutes. Add garlic, cook 1 minute, then drain fat. Stir in taco seasoning and ¼ cup water, simmer 2 minutes, and fold in the black beans.

Whisk together the sour cream, ½ cup of the enchilada sauce, and a pinch of salt until smooth.

Preheat oven to 375°F (190°C). Spread ½ cup enchilada sauce in a 9x13-inch baking dish. Layer 4 tortillas, half the beef mixture, half the sour cream mixture, ⅓ of the cheese, and ½ cup sauce. Repeat with another 4 tortillas, remaining beef, remaining sour cream mixture, another ⅓ cheese, and ½ cup sauce. Top with the last 4 tortillas, remaining sauce, and remaining cheese.

Cover with foil and bake 20 minutes. Uncover and bake 10 to 15 minutes more until bubbly and browned in spots.

Let rest 5 minutes, garnish with cilantro, slice, and serve hot.