1. Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. In a large skillet, heat olive oil over medium heat. Add the sliced onions and sugar, stirring occasionally until the onions become golden and caramelized, about 15-20 minutes.
3. Once the onions are caramelized, add the minced garlic and cook for another minute until fragrant. Remove from skillet and set aside.
4. In the same skillet, add the ground beef, breaking it apart with a spoon. Cook until browned, about 5-7 minutes. Drain excess fat if necessary.
5. Stir in the Worcestershire sauce, salt, and pepper to the beef mixture. Combine it with the caramelized onions.
6. Place the flatbreads on the prepared baking sheet. Evenly distribute the beef and onion mixture over each flatbread.
7. Top with shredded mozzarella cheese and bake in the oven for 8-10 minutes, or until the cheese is melted and bubbly.
8. Remove from the oven, garnish with fresh parsley if using, slice, and serve warm.

Pro tip: Keep an eye on the onions as they caramelize. Stir them occasionally to prevent burning and ensure they cook evenly—patience is key here.