Set your oven to 400°F (200°C) and place the taco shells upright inside a baking dish.
Cook the ground beef and diced onion in a skillet over medium heat until nicely browned, then drain off any excess fat.
Stir in the minced garlic and taco seasoning, cooking for 1 minute to combine the flavors.
Pour in the tomato sauce or salsa and let the mixture simmer gently for about 5 minutes.
Fill each taco shell evenly with the beef mixture, then top generously with shredded cheese.
Bake the assembled tacos in the oven for 10 to 15 minutes until the cheese is fully melted and the shells become crisp and golden.
Take the tacos out of the oven and add fresh toppings like shredded lettuce, tomatoes, red onion, and cilantro.
Serve the tacos warm, offering sour cream, guacamole, jalapeño slices, and hot sauce on the side as desired.
