Preheat your oven to 400°F and line a baking sheet with parchment paper. While the oven heats, finely dice the onion quarter, mince the garlic cloves, and finely chop the fresh parsley. Grate the Parmesan cheese fresh if you haven't already—this makes a noticeable difference in flavor and texture compared to pre-grated cheese. Having everything prepped and ready will make the mixing process quick and efficient.

Pour the milk into a large mixing bowl and add the Italian bread crumbs. Stir them together gently and let the mixture sit for 5 minutes—this allows the bread crumbs to absorb the milk and create a moist binder that will hold the meatballs together. This step is crucial for keeping the meatballs tender rather than dense and crumbly.

Add the ground beef, prepared onion, minced garlic, egg, salt, black pepper, grated Parmesan, chopped parsley, and red pepper flakes to the bread crumb mixture. Using your hands, gently fold and mix everything together until just combined—avoid overworking the mixture, as this can make the meatballs tough. I find that mixing with my hands lets me feel when the ingredients are evenly distributed without overdoing it.

Scoop the meat mixture onto the prepared baking sheet using a 2-tablespoon ice cream scoop or large spoon—this ensures uniform meatball sizes so they cook evenly. Wet your hands with cold water and gently roll each portion into a smooth 2-inch ball, rewetting your hands between rolls to prevent sticking. Space the meatballs about an inch apart on the sheet so heat can circulate around them for even browning.

Place the baking sheet in the preheated 400°F oven and bake for 18-20 minutes until the meatballs reach an internal temperature of 165°F when checked with a meat thermometer. Once cooked through, carefully tilt the baking sheet and use a slotted spoon to remove excess fat that has accumulated. I like to let the meatballs rest on the sheet for a couple minutes after baking—it helps them hold their shape better when transferred to sauce or a serving dish.