Combine Meatball Mixture. In a large bowl, combine ground chicken breast, gluten-free breadcrumbs, green onion, cilantro, garlic, ginger, and black pepper. Use a fork or clean hands to gently mix until just combined. Try to use a light touch here and don't over-mix. Remove to a plate lined with paper towels. Repeat with second half of the meatballs. Drain any fat or remaining oil from the pan.
Divide & Roll. Divide the mixture into 20 meatballs. (I use a medium cookie scoop for this!) Gently roll meatballs smooth between your hands.
Make Sauce. In a liquid measuring cup or small bowl, combine water, gluten-free tamari, pure maple syrup, rice vinegar, sesame oil, pepper, and cornstarch. Whisk until evenly blended.
Brown Meatballs. Heat 1 Tablespoon oil in a large skillet over medium heat. Working in batches (I cook 1/2 the meatballs at a time!), brown meatballs 2-3 minutes per side, just till golden brown. The meatballs will NOT be fully cooked through at this point. Set aside on a clean plate and repeat until all meatballs are browned, adding more oil, if needed.
Thicken Sauce. Give the sauce one more whisk to make sure there are no lumps of cornstarch at the bottom. Add the sauce to the pan and use a whisk to scrape up any browned bits from the bottom of the pan. Cook 2-3 minutes over medium heat until the sauce has thickened.
Coat & Cook Meatballs. Add all the browned meatballs back into the skillet and gently coat with sauce. Cover the pan and cook 6-8 minutes, stirring occasionally, or until the meatballs are fully cooked through. An instant read thermometer inserted in the center of the meatballs should reach at least 165 degrees F.
Serve & Store. Garnish with additional cilantro, green onions, or black pepper to taste, or top with sesame seeds or red pepper flakes! Store leftovers in an airtight container in the refrigerator 2-3 days. (The sauce will thicken even more in the fridge, but should loosen back up when re-heated.)