Prep the Oven: Preheat your oven to 375°F (190°C). Grease a 9x13 baking dish with a little oil or cooking spray.
Brown the Beef: In a large skillet, cook the ground beef and diced onion over medium-high heat until the beef is browned and onions are soft. Drain excess fat.
Season: Add minced garlic and taco seasoning to the beef. Stir in 1/2 cup of the enchilada sauce and 1 cup of the shredded cheese. This creates that "die-for" gooey filling.
Soften Tortillas: To prevent cracking, lightly heat tortillas in a pan with a drop of oil for 10 seconds per side, or microwave them in a damp paper towel for 30 seconds.
Assemble: Spread 1/2 cup of sauce at the bottom of the baking dish. Take a tortilla, add 2-3 tablespoons of the beef mixture, roll tightly, and place seam-side down in the dish.
Sauce and Cheese: Pour the remaining enchilada sauce over the top of the rolls. Sprinkle the remaining 2 cups of cheese generously over the sauce.
Bake: Place in the oven and bake for 20-25 minutes until the cheese is bubbly and starting to turn golden brown at the edges.
Rest and Serve: Let the casserole rest for 5 minutes (this helps the sauce set). Garnish with cilantro, sour cream, and jalapeños before serving.