Rub steaks with lime juice and ginger. Heat grill, broiler or cast iron grill pan. Grill fish for 3 minutes per side for medium rare steak. Grill bread to toast. In a small bowl mix together the mayonnaise, lime juice, horseradish and cayenne. Spread mayonnaise mixture on grilled bread and top with tuna steak, water cress and second slice of bread.
Recipe Courtesy of Food Network Kitchens