Preheat the oven to 400 degrees F and line a sheet pan with parchment paper.
Grate and squeeze the potato and apple. Working in batches, wrap in a kitchen towel or paper towel and squeeze over the sink until no more liquid comes out. You should have around 1/2 cup of shredded potato, and 1/2 cup of shredded apple.
In a small bowl, whisk the egg and add the oats. Let sit for a few minutes to hydrate.
In a large bowl, mix the ground turkey, potato, apple, grated cheddar, maple syrup, sage, fennel, salt, garlic powder, cinnamon, and black pepper. Mix gently until it's well combined. Now fold in the oat/egg mixture.
Form the mixture into 1.5 - 2 inch meatballs (about 20 total) and arrange on the prepared pan.
Bake for 20 - 25 minutes, or until lightly golden. If using an extra maple glaze, add the glaze the final 3 - 5 minutes of baking.
Remove from oven and rest. Let sit for 3- 5 minutes before serving. These do well stored in the fridge and re-heated for meal prep.