Step 1 Preheat the oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with nonstick cooking spray or a thin film of oil. This prevents sticking and makes serving and cleanup easier. Set the dish aside while you prepare the filling. Ensuring the oven is fully heated before the casserole goes in promotes even melting of the cheese and thorough heating of the layers.

Step 2 Place a large skillet over medium-high heat. Add the ground beef and diced onion. Cook for 7 to 9 minutes, breaking the beef into small crumbles with a wooden spoon or spatula, until the beef is fully browned and no longer pink and the onion has softened and turned translucent. Carefully drain any excess grease, leaving just a small amount in the pan for flavor. Thorough browning develops deep savory notes that form the foundation of the casserole.

Step 3 Sprinkle the taco seasoning over the beef and onion mixture. Pour in the cup of salsa. Stir well to combine, then reduce the heat to medium and let the mixture simmer for 2 to 3 minutes. This short simmer allows the seasoning to bloom and the salsa to meld with the beef, creating a cohesive, flavorful filling. Taste and add a pinch of salt or black pepper only if needed. Remove the skillet from the heat once the mixture is well combined and slightly thickened.

Step 4 Spread approximately half of the lightly crushed tortilla chips in an even layer across the bottom of the prepared baking dish. The chips should cover the surface without large gaps; gentle crushing (not powder) preserves texture while allowing the chips to layer neatly.

Step 5 Spoon half of the seasoned beef mixture evenly over the chip layer. Dollop half of the sour cream across the beef in small spoonfuls, then gently spread it as evenly as possible. Sprinkle half of the shredded Mexican-blend cheese over the sour cream layer. These alternating layers of chips, beef, sour cream, and cheese create the signature loaded texture and flavor distribution.

Step 6 Repeat the layering process: add the remaining crushed tortilla chips, followed by the rest of the beef mixture, the remaining sour cream, and the final portion of shredded cheese. The top should be covered generously with cheese so it melts into a bubbly, golden blanket. Press down very lightly with a spatula if needed to settle the layers, but avoid compacting them too firmly.

Step 7 Place the uncovered casserole in the preheated oven and bake for 20 to 25 minutes. The exact time depends on your oven; look for the cheese to be completely melted, bubbly, and beginning to turn golden in spots, and for the edges of the casserole to show gentle bubbling. The chips will soften slightly where they contact the hot filling while retaining some structure.

Step 8 Carefully remove the casserole from the oven and let it rest for 5 minutes. This brief cooling period allows the layers to set slightly, making scooping cleaner and preventing the filling from being overly runny.

Step 9 Scatter the diced tomatoes, sliced green onions, optional black olives, and optional chopped cilantro evenly over the hot, melted cheese. Adding these fresh ingredients after baking keeps them crisp, bright, and colorful rather than cooked and muted. The contrast between the hot, cheesy casserole and the cool, fresh toppings is one of the dish’s best features.

Step 10 Serve the casserole warm, scooping down through all the layers so each portion includes chips, beef, sour cream, cheese, and fresh toppings. Offer extra salsa, sour cream, or hot sauce on the side for those who want additional customization.