Preheat the Broiler
Preheat the oven to broil, positioning the rack about 8 inches from the heat source.
Cook the Beef
Heat a large ovenproof skillet over medium-high heat. Add the ground beef, taco seasoning, and about 1 cup of the chopped onion. Cook, stirring often, until the beef is browned and the onion turns translucent, about 6–7 minutes. Do not drain the mixture.
Add Salsa and Beans
Lower the heat to medium. Stir in the salsa and black beans. Allow the mixture to simmer for about 3 minutes, stirring occasionally, until the sauce thickens slightly and the flavors meld.
Build the Casserole
Gently fold in the tortilla chips until they’re evenly coated with the beef mixture.
Broil the Casserole
Sprinkle the shredded cheese over the top of the mixture. Transfer the skillet to the preheated oven and broil for 2–3 minutes, or until the cheese is melted and bubbly.
Add Toppings and Serve
Remove the skillet from the oven. Top the casserole with shredded lettuce, chopped tomatoes, sour cream, and the remaining diced onion. Serve immediately for best flavor and texture.