Combine spinach, onions, vinegar, Worcestershire, salt and peppers in blender or food processor and process until finely minced. Transfer mixture into a mixing bowl with ground chicken and bread crumbs; stir until well combined. Form into 12 golf ball-sized meatballs. Heat oil in a 12-inch nonstick skillet over medium-high heat. Arrange meatballs in skillet and cook for 6 to 8 minutes turning meatballs until browned and cooked through.