Preheat the oven to 400°F. Line a baking sheet with parchment paper for easy cleanup.
Combine ground turkey, breadcrumbs, crumbled feta, finely chopped onion, minced garlic, egg, parsley, oregano, and a pinch of salt and pepper. Use your hands to mix gently until just combined.
Shape the mixture into 1 to 1 1/4 inch meatballs and place them on the prepared baking sheet, leaving space between each.
Bake for 20 to 25 minutes, until the meatballs reach 165°F and are lightly golden on the outside, broiling for 1 to 2 minutes for extra color.
Make the tzatziki by grating the cucumber, squeezing out excess moisture, and stirring it into the Greek yogurt with lemon juice, minced garlic, and a pinch of salt and pepper.
Serve the warm meatballs with tzatziki on the side for dipping or spooned on top.
Notes
For extra flavor, add lemon zest to the meatball mix or substitute dill for parsley in the tzatziki.