Heat oven to 425ºF. In a medium bowl, stir together the nuts, panko, flour, 1/2 teaspoon of the salt,1/4 teaspoon of the pepper, and butter. Set aside.
Place a piece of aluminum foil on a baking sheet and brush it liberally with vegetable oil. Place the mahi mahi on the foil and sprinkle each fillet with the remaining salt and pepper on both sides. Bake for 5 minutes.
After 5 minutes, remove the fish from the oven and brush each fillet with the coconut milk. Divide the nut mixture among the tops of the 4 fillets, patting the mixture to spread and adhere to the fillets. Return to the oven and bake until the crust is golden brown, 5 more minutes.
Remove from the oven and allow to stand 10 minutes before serving.