1. Toast the black peppercorns and cloves together in a dry skillet over moderate heat until you see a puff or two of smoke. Remove from the heat and grind them together in an electric spice grinder or a clean coffee bean grinder. Transfer them to a heavy saucepan. Add the remaining ingredients and heat all together on medium heat. The mixture will begin to foam as it reduces. When it has reduced by approximately half, about 25 to 35 minutes, strain it through a fine-meshed strainer into a bowl. Reserve until needed.
Note: The rum can suddenly ignite. If it does, cover the pan to kill off the oxygen. Resume heating and reducing as before.
For the fish:
1. Just before serving the dish, preheat an oven to 450°F. Place the fish on a large platter and then paint the fish liberally on one side only.
2. Heat a large skillet (or two of them) until almost smoking hot.
3. Add some peanut oil and carefully add the fish, paint-side down. Shake the pan to avoid sticking. When the fish is quite dark, flip it over, degrease any excess oil and place the skillet in the oven. Bake the fish until just cooked through.
4. Lay the mangoes over the fish evenly.
5. Serve with a wedge of lemon or lime.
Note: Drizzle the prepared "pepper paint" out of a squirt bottle around the fish.