Preheat oven to 400°F (200°C). Arrange taco shells upright in a 9x13-inch baking dish, making sure they're snug enough to stand but not touching. Position them so they lean slightly against each other for support.

In a large skillet over medium heat, cook ground beef and diced onion for 6-8 minutes, breaking up the meat with a wooden spoon into small crumbles, until no pink remains and beef is evenly browned. Drain excess fat.

Add minced garlic and taco seasoning to the beef mixture, stirring constantly for 1 minute until fragrant. Pour in tomato sauce and simmer for 5 minutes, stirring occasionally, until mixture thickens slightly and liquid reduces.

Carefully spoon seasoned beef mixture into each taco shell, filling them about three-quarters full (approximately 2-3 tablespoons per shell). Avoid overfilling to prevent shells from cracking.

Generously sprinkle shredded cheese over each filled taco, covering the meat completely.

Place baking dish in preheated oven and bake for 10-15 minutes, until cheese is fully melted, bubbling, and just starting to turn golden at the edges. Shells should turn golden brown around bottom edges.

Remove from oven and let rest for 2-3 minutes to prevent lettuce from wilting. Top each taco with fresh lettuce, diced tomatoes, red onion, and cilantro. Serve immediately with sour cream, guacamole, and desired toppings.