Place garlic, onion, and parsley in a food processor. Pulse several times, wiping down the sides of the bowl, until all the ingredients are puréed. Transfer to a medium sized mixing bowl, along with the ground beef. Sprinkle the salt, coriander, cumin, ginger, black pepper, and cayenne on top and mix thoroughly with your hands.
Form mixture into 18 meatballs, using a tablespoon sized cookie scoop. Thread onto metal skewers and transfer to a baking sheet; flatten meatballs slightly. Cover wth plastic wrap and refrigerate for at least 2 hours and up to overnight.
Preheat an outdoor grill to medium-high; oil grates. Place skewers on the grates, close lid, and cook for about 4 minutes per side until meat is cooked through (160 degrees F in the thickest part.)
Serve meatballs on flatbread with some of the Tzatziki sauce and fresh chopped parsley.