Preheat the oven to 400°F (200°C). Arrange the hard taco shells upright in a baking dish so they won’t tip over.

Heat a skillet over medium. Add the ground beef and diced onion. Cook, breaking the meat into small pieces, until the beef is evenly browned.

Drain excess fat from the skillet to prevent greasy filling. Return the pan to the heat and stir in the minced garlic and taco seasoning. Cook for about 1 minute until fragrant.

Pour in the tomato sauce or salsa, stir to combine, then reduce the heat and simmer gently for 4 to 5 minutes so the flavors meld. Season with salt and pepper to taste.

Spoon the beef mixture into each taco shell, filling them evenly but not packing too tight.

Generously sprinkle shredded cheese over the top of each filled shell.

Place the baking dish in the preheated oven and bake for 10 to 15 minutes, or until the cheese has melted and the edges of the shells are lightly golden.

Remove from the oven and top with shredded lettuce, diced tomatoes, red onion, and cilantro if using. Serve immediately with sour cream, guacamole, jalapeños, or hot sauce on the side.