Preheat oven to 400°F (200°C). Spray a 9x13 baking dish lightly with cooking spray.

In a large skillet over medium-high heat, cook ground beef and onion until meat is browned and onions are soft. Drain excess grease.

Add minced garlic and cook for 1 minute until fragrant.

Stir in taco seasoning and tomato sauce (or salsa). Simmer on low for 3-5 minutes until thickened.

Optional: Spread a thin layer of refried beans inside the bottom of each taco shell.

Stand the taco shells upright in the prepared baking dish. Fill each shell with the beef mixture.

Generously sprinkle shredded cheese over the tacos.

Bake for 10-12 minutes, or until cheese is melted and bubbling and shells are golden brown.

Remove from oven and immediately top with lettuce, tomatoes, red onion, and cilantro before serving.