In a large bowl combine the beef, onion, egg, 1/2 cup mint, lemon juice, cumin, and cinnamon. Use your hands to mix it up until the ingredients are evenly distributed. Shape mixture into 20 2-inch balls. Flatten balls slightly.
In a large cast iron or heavy skillet heat the oil over medium heat. Add half of the meatballs in a single layer and cook until no longer pink in the center (160°F), about 8 minutes, turning occasionally to brown evenly. Remove and cover to keep warm. Repeat with remaining meatballs.
Serve topped with feta and additional mint, if desired.