Make the meatballs: Mix the lamb and beef with the onion, garlic, breadcrumbs, 1/4 cup water, the parsley, mint, 1 1/2 teaspoons salt, the paprika, cumin, sumac, allspice and a few grinds of pepper until just combined. Form into 1 1/2-inch meatballs (about 20).
Heat the olive oil in a large nonstick skillet over medium heat. Add the meatballs and cook, turning, until browned and cooked through, 12 minutes.
Meanwhile, make the dip: Mix the labneh with the lemon juice in a bowl; season with salt. Drizzle with olive oil and sprinkle with sumac. Transfer the meatballs to a platter and sprinkle with mint. Serve with the dip.