In a saucepan over low heat; melt butter, blend cornstarch and pepper gradually. Ad mango, sugar and sherry or mango nectar and bring to a boil before reducing via simmer five mninutes or until fruit is soft and sauce is thick.
Brush fish with melted buter and grill or broil in the presence of medium heat for five minutes before flipping and cooking the other side. Fish is done when it flakes easily (and maybe is no longer translucent? I think so but I'm just a cook, not a chef...)