01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or oil.
02 - Brown ground beef in a large skillet over medium-high heat for about 5 minutes, breaking it up. Drain excess fat if needed.
03 - Add chopped onion and cook 3–4 minutes until softened. Stir in garlic, cumin, chili powder, smoked paprika, salt, and pepper. Cook 1 minute until fragrant.
04 - Stir in ½ cup enchilada sauce into the beef mixture until combined, then remove from heat.
05 - Spread ½ cup enchilada sauce evenly over the bottom of the baking dish. Layer 5 tortillas over the sauce, tearing to fit if necessary.
06 - Spread half of the beef mixture atop the tortillas, then sprinkle with 1 cup cheddar and ½ cup Monterey Jack cheese.
07 - Repeat with remaining tortillas, beef, ½ cup enchilada sauce, and the rest of the shredded cheeses.
08 - Drizzle remaining ½ cup enchilada sauce over the top and sprinkle with any reserved cheese.
09 - Cover dish with foil and bake for 20 minutes. Remove foil and continue baking uncovered for 10–15 minutes until cheese is bubbly and golden.
10 - Let the casserole rest 5–10 minutes before slicing. Garnish with sour cream, cilantro, diced avocado, and sliced green onions, if desired. Serve warm.