Combine first 5 ingredients; rub over tuna. Cover and chill 30 minutes. Combine wasabi and water. Let stand 5 minutes. Stir in mayonnaise, sour cream, juice, and soy sauce.
Prepare grill to high heat.
Place steaks on grill rack coated with cooking spray; grill 5 minutes on each side or until desired degree of doneness. Serve with wasabi cream; sprinkle with onions.