In a bowl combine onion and 3 cloves of garlic. Squeeze the water out of the spinach until completely dry. Separate the spinach as you add it to garlic and onion. Add feta, chicken and grill seasoning to the bowl and a liberal drizzle of extra-virgin olive oil. Mix the meat with veggies and feta and form 18 (1 1/2-inch) meatballs. Place the meatballs on a nonstick cookie sheet and bake 10 to 12 minutes until they are golden and juices run clear.
Place yogurt, garlic, cucumber, dill, lemon juice, cumin, coriander and a little salt in food processor and process until smooth. Adjust seasonings and transfer to a serving bowl. Serve meatballs alongside sauce with toothpicks.