Cook onion and garlic in olive oil with 1/2 teaspoon each of salt and pepper in a large skillet over medium heat until onion is softened, about 6 minutes.
Lightly beat egg in a large bowl, then combine with chicken, 1 tablespoon tomato paste, breadcrumbs, and parsley.
Add the onions and garlic and form 12 meatballs (or more if you like 'em tiny) and arrange in a mini muffin tin or glass bakeware.
Bake at 400° in upper third of oven until meatballs are just cooked through, 10 to 20 minutes depending on size.
Stir into sauce to keep warm if serving with pasta but they can be eaten as is if you prefer.