Preheat oven to 400°F (200°C). Line a baking sheet with parchment paper or lightly grease it.

Heat olive oil in a skillet over medium heat. Add diced onion and cook until soft, then stir in garlic.

Add ground beef and cook until browned. Drain excess fat if needed.

Stir in taco seasoning and diced tomatoes. Simmer 3–5 minutes, then remove from heat.

Arrange taco shells upright on the baking sheet. Sprinkle a small amount of cheese into the bottom of each shell.

Fill shells with meat mixture, then top with remaining cheese.

Bake for 10–12 minutes until shells are golden and cheese is melted.

Remove from oven, garnish with lettuce, tomatoes, sour cream, and serve immediately.