1. Preheat your oven to 200°C (400°F).

2. In a large bowl, combine the ground beef, chopped onion, garlic, egg, breadcrumbs, dried oregano, salt, black pepper, and red pepper flakes (if using). Mix everything together with your hands or a wooden spoon until the ingredients are just combined.

3. Divide the meat mixture into 12 equal portions and shape each one into a meatball. Place the meatballs on a baking sheet lined with parchment paper.

4. Bake the meatballs in the preheated oven for 18-20 minutes, or until they are cooked through.

5. While the meatballs are baking, prepare the pickled red onion. Place the thinly sliced red onion in a bowl. In a small bowl, whisk together the white vinegar, granulated sugar, and water until the sugar is dissolved. Pour the vinegar mixture over the red onion slices and let it sit at room temperature for at least 30 minutes to allow the onions to pickle.

6. Once the meatballs are done, remove them from the oven and let them cool for a few minutes.

7. Top each pizza kit with 3-4 meatballs, a spoonful of pickled red onion, and a sprinkle of shredded mozzarella cheese (if using). Bake in the oven for an additional 5-7 minutes, or until the cheese is melted and bubbly.