Preheat oven to 425º F. Line a baking sheet with parchment paper or foil and set aside.
In a small bowl, stir together breadcrumbs and milk. Let sit 5 minutes or until liquid has been soaked up.
In a large bowl, combine ground beef and lamb, breadcrumb mixture, ½-cup onion, olives, egg, Greek seasoning, oregano, salt, pepper and lemon zest. Using your hands to mix works best to incorporate well.
Roll the meat mixture into 18 balls, roughly 1-½ inches in diameter. Set on prepared baking sheet 1-inch apart. Optional: let meatballs sit at room temp 15-30 minutes before baking for more even cooking.
Bake 12-14 minutes, or until outsides are browned and internal temp reaches 165º. Remove from oven and serve immediately.
While Greek meatballs are baking: Cut remaining red onion into thin slices. Cut cucumber into quarters lengthwise, then into smaller cubes. Cut tomatoes in half. If feta is in a block cut into smaller cubes.
Spread veggies onto a serving dish and top with baked meatballs and tzatziki sauce. Serve with toothpicks for an easy party appetizer.
Be sure to soak the breadcrumbs in milk before adding to the meat. This is an important step for proper texture.
For keto meatballs, substitute bread crumbs with almond meal.
The extra fat in 85% lean ground beef will create much juicier meatballs and better flavor.
Roll the meat mixture into even balls. If meatballs are different sizes they will bake unevenly.
Line baking sheet with parchment paper or foil for easy cleanup.
Overbaking will dry out meatballs.
Serve immediately for best texture and flavor. If prepping ahead of time, keep warm under foil until ready to serve.