Combine the first 9 Meatball ingredients (Gruyere through red pepper flakes) in a large bowl. Then add the ground chicken and mix with your hands until the ingredients are evenly distributed (work quickly and gently as you do not want to over-work the chicken). Using a 2 to 3-tablespoon scoop, portion the mixture into 22-28 meatballs and place them at least 1 inch apart on a parchment-lined baking sheet. Place the entire baking sheet in the fridge for the meatballs to set-up for approximately 30 minutes. (See TIP A below for make-ahead and freezing instructions.)
Using a silicone brush, lightly brush the top of each meatball with olive oil. Then transfer the sheet pan of meatballs from the fridge to the top-most rack of a pre-heated 400° oven. Bake for 25-30 minutes or until the meatballs are cooked through and just slightly golden on top and bottom (turn-on your oven's broiler for the last 2-3 minutes if desired).
Whisk the Sauce ingredients together in a small bowl and set aside.
Transfer the finished meatballs to a serving plate or bowl and drizzle with the Cider Yogurt Sauce. Serve the remaining sauce on the side.
TIP A: These meatballs are a great make-head meal. After portioning them into meatballs in Step One, put the baking sheet into the freezer (instead of the fridge) for 30-60 minutes. Once the meatballs have firmed-up, transfer them to an airtight container between pieces of parchment paper. Store in the freezer for up to 2 weeks. Defrost in the refrigerator before baking.