Prep the oven. Heat to 425°F (220°C).
Line a sheet pan with parchment and brush or spray lightly with olive oil.
Make the tzatziki first. Grate the cucumber, then squeeze out excess water with a clean towel. Stir it into the yogurt with garlic, lemon juice, dill, olive oil, salt, and pepper. Adjust seasoning.
Chill while you make the meatballs so the flavors meld.
Grate the onion. Use the large holes of a box grater over a bowl to catch the juices. This keeps the turkey moist without large onion chunks.
Mix the meatball base. In a large bowl, combine turkey, grated onion with juices, garlic, breadcrumbs, feta, egg, parsley, dill, oregano, lemon zest, cumin, salt, and pepper. Use a fork or your hands to mix gently until just combined.
Avoid overmixing.
Shape the meatballs. Scoop about 2 tablespoons per meatball and roll lightly with damp hands. You should get 20–24 meatballs. Place evenly on the prepared sheet pan and brush tops with a little olive oil for browning.
Bake. Cook for 12–14 minutes until almost done, then switch to broil for 2–3 minutes to brown the tops.
Internal temperature should reach 165°F (74°C).
Rest and serve. Let meatballs rest 3–5 minutes. Serve with a generous spoon of tzatziki, fresh veggies, and your choice of pita or grains. Finish with a squeeze of lemon.