Peel and chop the yams, then place them in a large pot with salted water. Boil until tender, about 25 minutes. Drain and set aside. Prep the vegetables.
Heat the olive oil in a pan on low, add the prepared salted cod, and gently fry for a few minutes.
Raise the heat to medium and add half of the black pepper, onion, garlic, scotch bonnet, scallions, bell peppers, christophene, okra, pumpkin, and tomato. Cover and cook on low heat for 20 minutes, stirring every 5 minutes until everything is tender.
Heat the vegetable oil in a frying pan over medium heat and fry the sliced plantain until golden on both sides, about 4–5 minutes. Set aside.
Test the cooked yam with a paring knife, drain fully if any liquid remains, and add the remaining black pepper, butter, and evaporated milk. Mash until smooth. Stir in three-quarters of the medium and aged cheddar. A small amount of freshly grated nutmeg may be added if desired.
Brush the baking dish with melted butter or olive oil. Add a layer of mashed yam, smoothing to settle it evenly. Spoon on two-thirds of the stewed saltfish mixture, followed by the fried plantain slices.
Add another layer of mashed yam, level it with a spoon, then top with the remaining cheeses and the rest of the stewed saltfish.
Place the dish on the middle rack of a preheated 375°F (190°C) oven. Bake uncovered for 1 hour. Allow the pie to cool for 20–30 minutes before serving so it can set.