Preheat the oven to 375°F (190°C).
Heat a large skillet over medium-high heat and add the ground beef. Cook, breaking up with a spatula, until no pink remains and the internal temperature reaches 165°F, about 6–8 minutes.
Drain any excess fat from the skillet using a colander or by tipping and blotting with paper towels.
Return the beef to the skillet and stir in the tomato sauce and taco seasoning. Simmer over medium-low heat, stirring occasionally, until the mixture thickens, about 2–4 minutes.
Stand the taco shells upright, packed tightly, in a 9×13-inch baking dish.
Spoon the seasoned beef into each shell and sprinkle the shredded cheese evenly over the filled tacos.
Bake for 8–10 minutes, until the cheese is melted and the shells are crisp.
Remove from the oven and top each taco with shredded lettuce, pico de gallo, corn salsa, sour cream, and guacamole as desired, or let guests add their own toppings.
