Combine all ingredients except oil in a blender or food processor and blend until pureed. Add oil slowly in a stream until fully combined. Set aside.
Heat spinach on a microwave-safe plate for one minute; set aside to cool. Transfer to double layer of paper towels; fold over and squeeze excess moisture from spinach.
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.
Spread pesto onto both halves of each slider rolls, add desired toppings and one meatball per roll. Serve immediately.
If desired, top with lettuce, tomato OR red onion.
Slightly flatten each meatball for quicker cooking and easier eating!