Preheat the oven to 350F.
In a pan over medium heat, add the olive oil, onions and garlic, stirring frequently. You want the onions to be very light in color and softened, which should take about 5-7 minutes. Set aside to cool.
Once the onions are cool, combine them with all of the remaining ingredients (chicken, breadcrumbs, egg, garlic powder (if using instead of fresh garlic), dill, mozzarella, lemon zest, oregano, parsley, celery salt, salt, and lemon pepper).
Use your hands to combine the mixture and make sure everything is distributed equally throughout the mix.
Form into 16 equal sized meatballs and place on a parchment lined baking sheet. Bake for 15 minutes, flipping the meatballs at the 10 minute mark. That will help you get a nice sear on both sides!
While the meatballs bake, combine all of the ingredients for the tzatziki in a small bowl. Stir to combine.
Serve the meatballs with rice, pita, salad or plain! But no matter what you serve it on, definitely top them with tzatziki!