In a large bowl, combine the egg, onion, raisins, walnuts, mint, 1 tablespoon oil, garlic and salt. Crumble meat over mixture and mix well. Shape into 12 balls.
In a large skillet, brown meatballs in remaining oil in batches. Drain on paper towels. Place on a greased rack in a shallow baking pan. Bake, uncovered, at 350° for 13-15 minutes or until a thermometer reads 160°.
Meanwhile, in a small bowl, combine the sauce ingredients. Line buns with cucumber slices and arugula; fill with meatballs and drizzle with sauce.