To make the tzatziki sauce, combine Greek yogurt, cucumber, garlic, dill, lemon juice, lemon zest and mint in a small bowl; season with salt and pepper, to taste. Drizzle with olive oil. Refrigerate for at least 10 minutes, allowing the flavors to meld; set aside.
Preheat oven to 400 degrees F. Lightly oil a 9×13 baking dish or coat with nonstick spray.
In a large bowl, combine ground turkey, breadcrumbs, onion, egg yolks, garlic, lemon zest, oregano, cumin, salt and pepper, to taste. Using a wooden spoon or clean hands, stir until well combined. Roll the mixture into 1 1/4-to-1 1/2-inch meatballs, forming about 18-20 meatballs.
Place meatballs onto the prepared baking dish and bake for 18-20 minutes, or until all sides are browned and meatballs are cooked through.
Serve immediately with tzatziki sauce, garnished with dill and lemon zest, if desired.