Season the fish with salt and pepper. Combine the yogurt and 1 tablespoon parsley in a shallow baking dish and add the fish, turning to coat. Marinate the fish for 15 minutes at room temperature or for 1 hour in the refrigerator.
Meanwhile, combine the reserved yogurt and remaining parsley in a small bowl and mix well.
Thread the skewers, alternating the fish, tomatoes, and scallions.
Prepare a stove-top griddle or outdoor grill. Grill the kebabs 3 to 4 minutes per side, or until opaque. Serve with a dollop of the yogurt sauce.