Preheat and prep: Heat your oven to 400°F (200°C).
Line a rimmed baking sheet with parchment or lightly oil it to prevent sticking.
Mix the base: In a large bowl, whisk the egg, Greek yogurt, garlic, parsley, Italian seasoning, onion powder, red pepper flakes (if using), salt, and pepper until smooth. Stir in the Parmesan and breadcrumbs.
Add the meat: Gently fold in the ground turkey or chicken with a spatula or your hands. Mix just until combined.
Overmixing makes meatballs tough.
Shape: Lightly oil your hands. Roll into 18–22 meatballs, about 1.5 inches each. Place them on the prepared sheet with a little space between.
Brush and bake: Brush the tops with a touch of olive oil.
Bake for 14–16 minutes, until just cooked through (internal temp 165°F/74°C).
Optional broil: Turn on the broiler and brown the tops for 1–2 minutes for a light crust. Watch closely to prevent burning.
Finish and serve: If using, drizzle with the quick garlic-butter-parsley mix and a squeeze of lemon. Sprinkle extra Parmesan.
Serve hot with your favorite sides.