Add 1 pound ground chicken, ¼ cup minced white onion, 3 cloves garlic, 1 large egg, ½ cup finely crumbled feta cheese, 2 teaspoons Greek seasoning, and ½ teaspoon sea salt to large mixing bowl. Stir ingredients together with large wooden spoon until ingredients are fully combined and well-blended.
Use cookie scoop to portion ground chicken mixture into meatballs of desired size. Quickly roll each meatball between palms of hands to shape into smooth, round ball, then place meatball on large plate and set aside.
Repeat process with remaining ground chicken mixture until entire mixture has been formed into meatballs. Note: Mixture will yield approximately 14 to 20 meatballs depending on size of cookie scoop.
Heat large cast-iron skillet over medium heat. When pan is warm, add 1-2 tablespoons neutral oil to skillet, just enough to coat bottom of skillet in thin layer of oil. Continue heating skillet until oil is hot and shimmering. Note: While skillet warms up, line remaining plate with paper towels and set aside.
When oil is hot, place meatballs in skillet, working in batches as needed to avoid overcrowding pan. Sauté meatballs 1 to 2 minutes or until golden brown on bottom, then flip meatballs over. Sauté meatballs on other side 1 to 2 minutes more.
Once meatballs are golden brown on both sides, and internal temperature of meatballs reaches 160° Fahrenheit, remove meatballs from skillet and transfer to plate lined with paper towels. Let meatballs rest 5 minutes.
Repeat process with any remaining meatballs until all meatballs have been cooked through, letting each batch rest on paper towels 5 minutes.
Once all meatballs have been cooked and rested, transfer meatballs to serving bowls or plates. Pair meatballs with hummus, tzatziki, chopped fresh dill, diced cucumber, halved grape tomatoes, lemon rice, and/or lemon wedges as desired and serve immediately.