Preheat your oven to 400°F and line a baking sheet with parchment paper or lightly oil it for easy cleanup. While the oven heats, mince your garlic and onion, chop your fresh parsley, and grate your Parmesan cheese. Having everything prepped and ready makes mixing the meatball mixture quick and prevents the egg from starting to cook the meat while you're still prepping ingredients.

In a large bowl, combine the ground beef with the egg, breadcrumbs, Parmesan cheese, minced onion, minced garlic, salt, black pepper, fresh parsley, and red pepper flakes. Mix gently with your hands until just combined—don't overmix, as this makes the meatballs dense and tough. I find that using my fingertips and folding the mixture rather than aggressively stirring keeps them tender and light.

Scoop the mixture using a small ice cream scoop or your hands and roll into 1½-inch balls, placing them on the prepared baking sheet about an inch apart. Bake for 8-10 minutes, then flip each meatball over and continue baking for another 8-10 minutes until the outside is lightly browned and cooked through (about 17-20 minutes total). The meatballs don't need to be completely cooked at this stage since they'll finish cooking in the sauce.

Transfer the baked meatballs to a pot or skillet with marinara sauce and simmer gently for 10 minutes. This allows the meatballs to absorb the flavors and ensures they're fully cooked through while staying juicy. The sauce also prevents them from drying out during the final cooking stage.