Preheat your oven to 400°F, ensuring the rack is in the upper third of the oven for optimal cooking.
Mix the meatball ingredients in a large bowl. Combine the ground chicken, breadcrumbs, onion, parsley, Pecorino (or Parm), salt, pepper, minced garlic, fennel seeds, whisked egg, and lemon zest. Do this gently with your hands to keep the mixture light.
Form the mixture into meatballs, about the size of a lacrosse ball, and place them on a foil-lined baking sheet, spaced a few inches apart.
Prepare the sauce mixture by combining a spoonful of pizza sauce with olive oil in a small bowl. Brush this mixture over each meatball.
Bake for 15 minutes, then remove from the oven. Top each meatball with a spoonful of pizza sauce and a slice of fresh mozzarella.
Broil for 3 to 5 minutes or until the cheese is bubbly and golden. Meanwhile, heat the remaining pizza sauce in a small saucepan.
Serve the meatballs with a dollop of the warmed sauce, accompanied by a side of your choice, such as a raw Tuscan kale salad.