In a large bowl, combine ground meat, garlic, onion, parsley, mint, oregano, olive oil, salt and pepper. Mix well.
Add egg and breadcrumbs. Mix until thoroughly combined.
Preheat oven to 375 F. Spray 2 cookie sheets with non stick spray.
Form into meatballs and place on the cookie sheets.
Bake for 15 minutes. Turn meatballs. Bake for another 20 minutes.
Meanwhile, add the lemon juice, mint, salt and pepper to the yogurt in a bowl.
Whisk well. Garnish with more sumac.
Serve meatballs with the mint yogurt sauce.