Preheat oven to 400°F and place a rack in the upper third of your oven.
In a skillet over medium heat, cook onion and garlic until the onion is translucent.
Remove from heat and cool slightly.
Combine onion mixture, ground chicken, egg yolk, 1/4 cup tomato sauce, parsley and spices in a large bowl.
Form meatballs by taking 1 tablespoon of the meat mixture and rolling into a ball.
Place the formed meatballs into a large baking dish about one inch apart.
Top meatballs with remaining tomato sauce.
Bake 15 minutes.