INGREDIENTS 1 cup  precooked shredded potatoes, (see Note) 1  shallot, finely chopped 1 Tbsp  prepared horseradish 1 tsp  Dijon mustard 1/2 tsp  garlic salt 1/4 tsp  freshly ground pepper 1 1/4 pounds  mahi-mahi, skin removed, cut into 4 portions 4 tsp  reduced-fat mayonnaise 1 Tbsp  …