In a glass casserole place the fish, onion, tomatoes, chili, salt, cayenne pepper and oregano. Cover with citrus juice.
Let sit covered in the refrigerator for 3 1/2 hours then ladle more of the marinade over the fish.
In a nonstick pan heat oil on high heat and cook fish on each side for 3 minutes.
Mix cornstarch and water in small bowl until smooth to create a slurry.
Transfer remaining marinade into a food processor, blend and bring to a boil in a heavy-bottomed saucepot and thicken with the slurry.
Pour sauce over snapper. Mix cilantro, tomato, and mango to create salsa and spoon over snapper.
Serve while hot.
