Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
Squeeze excess moisture from the grated apple using a clean kitchen towel to prevent soggy meatballs.
In a large mixing bowl, combine ground chicken, grated apple, breadcrumbs, onion, egg, sage, salt, pepper, and cinnamon.
Mix by hand until just combined, taking care not to overwork the meat.
Form the mixture into approximately 16 to 20 meatballs, about 1.5 inches in diameter.
Arrange meatballs on the prepared baking sheet with at least 1 inch of space between each.
Lightly brush the tops of the meatballs with olive oil for browning.
Bake for 18 to 20 minutes or until the internal temperature reaches 165°F (74°C).
Allow meatballs to rest for 5 minutes before serving to retain juices.