For marinade, in a small bowl combine soy sauce and rice vinegar; set aside.

Thaw scallops, if frozen. Rinse scallops, pat dry with paper towels. Trim root end and green tops of leek. Rinse leek thoroughly to remove any grit. Cut the one small leek lengthwise into quarters; insert a wooden pick crosswise through each leek quarter to hold layers together when grilling. Or, trim the baby leeks.

Place leeks, scallops, and scallions in a plastic bag set in a shallow bowl. Add marinade. Seal bag; turn to coat scallops and vegetables. Marinate in refrigerator for 30 minutes.

Remove scallops, leeks, and scallions from bag. Discard marinade. For a charcoal grill, place leeks, scallops, scallions, and lime halves (cut side down) on the rack of an uncovered grill directly over medium coals. Grill for 8 to 10 minutes or until scallops are opaque, turning scallops and vegetables occasionally. Remove any scallions from grill rack before they overbrown. (For a gas grill, preheat grill. Reduce heat to medium. Place leeks, scallions, and scallops on grill rack over heat. Grill as above.)

To serve, transfer leeks and scallions to serving plates. Top with scallops. Using grilling tongs, remove limes from grill and squeeze over scallops. Sprinkle with black sesame seeds, if desired. Serve with melted butter. Makes 4 servings.

If desired, melt butter in heat resistant bowl or ramekin on grill alongside scallops and leeks. Makes: 4 servings