In a large mixing bowl, combine the ground beef, breadcrumbs, grated Parmesan cheese, chopped parsley, minced garlic, egg, Italian seasoning, salt, black pepper, and red pepper flakes.
Mix the ingredients with your hands or a spoon until well combined, but don't overmix, as this can make the meatballs tough.
Preheat your oven to 400°F (200°C).
Use your hands to form the mixture into meatballs, roughly 1 inch in diameter, and place them on a baking sheet.
In a large skillet, heat the olive oil over medium heat. Once hot, add the meatballs in batches, being careful not to overcrowd the pan. Brown each meatball on all sides for about 5-7 minutes.
After browning, transfer the skillet to the preheated oven or transfer the meatballs to an oven-safe dish and bake for an additional 10-15 minutes until fully cooked (internal temperature of 165°F/74°C).
While the meatballs are baking, warm the marinara sauce in a saucepan over low heat.
Once the meatballs are cooked, remove them from the oven and let them cool for a few minutes.
Serve the meatballs in bowls topped with warm marinara sauce and an extra sprinkle of Parmesan cheese and parsley if desired.
Notes
Be careful not to overmix the meatball mixture to keep them tender.