In a bowl, whisk together lemon juice, olive oil and soy sauce until blended. Stir in onion and pepper. Add lamb and toss to coat. Cover and refrigerate for several hours or overnight, stirring occasionally.
Bring lamb to room temperature 1 hour before grilling. Thread cubes onto 6 metal skewers and set aside. In a small bowl, whisk together melted butter, yogurt, cumin and salt. Grill meat 4 to 6 inches away from the fire, turning occasionally and basting with yogurt mixture 2 to 3 times while cooking to desired doneness (best when rare to medium-rare).
Tuck lamb into warmed pitas and sprinkle in scallions, tomato and mint. Serve any remaining yogurt sauce on the side.