Preheat the oven: Heat the oven to 400°F (200°C). Line a baking sheet with parchment paper or foil and lightly grease it.
Combine the ingredients: Add the ground beef, breadcrumbs, Parmesan, egg, garlic, onion, salt, and black pepper to a large bowl.
Add moisture if needed: If the mixture feels unusually dry, add 1 tablespoon of milk or water. Add another tablespoon only if necessary.
Mix gently: Use your hands or a fork to combine everything just until the ingredients are evenly distributed. Do not overmix.
Shape the meatballs: Divide the mixture into about 24 portions. Gently roll each portion into a ball about 1½ inches wide.
Arrange on the baking sheet: Place the meatballs on the prepared baking sheet, leaving a little space between each one.
Bake: Bake for about 15 minutes, depending on their size.
Check the temperature: Insert an instant-read thermometer into the center of the largest meatball. Ground beef should reach 160°F (71°C).
Rest briefly: Remove the meatballs from the oven and let them rest for a couple of minutes.
Serve: Serve immediately with warm marinara sauce and Parmesan, or use the meatballs for spaghetti, meatball subs, rice bowls, or appetizers.