Heat olive oil in a skillet over medium heat. Add the chopped onion and cook, stirring occasionally, until caramelized, about 15-20 minutes.

Add minced garlic and cook for an additional 1-2 minutes until fragrant. Remove from heat and let cool slightly.

In a large bowl, combine ground beef, caramelized onion mixture, breadcrumbs, Parmesan cheese, egg, oregano, salt, black pepper, and red pepper flakes (if using). Mix until just combined, being careful not to overmix.

Preheat the oven to 400°F. Line a baking sheet with parchment paper.

Form the meat mixture into 1-inch meatballs and place them on the prepared baking sheet.

Bake meatballs in the preheated oven for 20-25 minutes or until cooked through and browned.

While the meatballs are baking, heat marinara sauce in a saucepan over medium heat.

Once meatballs are done, add them to the marinara sauce and simmer for 5 minutes to combine flavors.

Serve hot, garnished with fresh parsley.