Prepare the Beef Filling : In a skillet over medium heat, cook ground beef with onions and garlic until beef is browned and onions are soft, about 5–7 minutes. Drain excess fat and stir in chili powder, cumin, smoked paprika, salt, black pepper, diced tomatoes, and black beans if using. Cook for 2–3 minutes until well combined.
Assemble the Enchiladas : Spread ½ cup of enchilada sauce on the bottom of the crockpot. Place a few tablespoons of beef filling in the center of each tortilla, sprinkle with a little cheese, and roll tightly. Arrange the rolled tortillas seam-side down in the crockpot. Pour remaining enchilada sauce evenly over the top and sprinkle remaining cheese.
Cook in Crockpot : Cover and cook on low for 4–5 hours or high for 2–3 hours until heated through and cheese is melted.
Serve : Garnish with fresh cilantro if desired. Serve hot with rice, beans, or a side salad.