Preheat oven to 400 degrees then line a half sheet pan with foil and place a cooling rack on top. Spray the cooling rack with nonstick spray then set aside.
In a large bowl, stir together red onion, garlic, egg white, bread crumbs, and ranch dip mix. Add ground chicken then mix with your hands until just combined. Divide mixture into quarters, then each quarter into 4 balls. Press a cube of feta into the center of each ball then close chicken around cheese to seal. Place meatballs onto the prepared sheet pan then bake for 16-18 minutes, or until cooked through.
Meanwhile, melt butter in a large skillet over medium heat then add buffalo wing sauce and stir to combine. Add cooked meatballs to the skillet then use a spoon to coat in sauce. Place a lid on top then turn heat to low to keep warm.