Preheat your oven’s broiler or set to bake at 400–425 °F.
In a medium bowl, mix ground lamb with onion, garlic, herbs, egg, breadcrumbs, salt, pepper (and optionally feta).
Form into golf‑ball sized meatballs, arranging them about 2 inches apart on a lined baking sheet.
Broil: Cook under the broiler for 5–7 minutes, turning halfway, until browned and cooked through. Or bake at 425 °F for 8–10 minutes, then optionally broil to crisp.
While meatballs cook, prepare tzatziki by combining yogurt, cucumber (well-drained), garlic, dill, mint, lemon juice, salt, and pepper.
Serve meatballs hot with tzatziki for dipping or drizzling.