Preheat your oven to 400°F (200°C). Arrange the hard taco shells upright in a baking dish.

Heat a skillet over medium. Add the ground beef and diced onion. Cook, breaking the meat up, until the beef is fully browned and the onion is soft. Season with a pinch of salt and pepper.

Carefully drain any excess fat from the pan. Return the skillet to medium heat. Stir in the minced garlic and taco seasoning. Cook for about 1 minute until fragrant.

Pour in the tomato sauce or salsa and stir. Let the mixture simmer gently for 4 to 5 minutes. Taste and adjust seasoning.

Spoon the beef mixture evenly into each prepared taco shell. Sprinkle the shredded cheese over the top.

Bake in the preheated oven for 10 to 15 minutes. Watch for bubbling cheese and lightly golden shells.

Remove from the oven and top the tacos with shredded lettuce, diced tomatoes, red onion, and chopped cilantro as desired.

Add sour cream, guacamole, jalapeño slices, or hot sauce on the side. Serve warm.