1. Preheat the oven to 375°F and lightly grease a baking dish.

2. Heat the olive oil in a skillet over medium heat, then cook the diced onion until softened.

3. Add the ground beef and cook until browned, breaking it apart as it cooks. Drain excess grease if needed.

4. Stir in the garlic, taco seasoning, tomato sauce, salt, and black pepper. Simmer for 2 to 3 minutes.

5. Arrange the taco shells upright in the baking dish.

6. Spoon the beef mixture evenly into each taco shell.

7. Top each taco with cheddar cheese and mozzarella cheese.

8. Bake for 10 to 12 minutes, until the cheese is melted and bubbly.

9. Remove from the oven and finish with diced tomatoes and chopped cilantro.

10. Serve immediately while hot and crispy.