Have a bowl or pan of water ready before you start, as you’ll need to put each peeled yam piece straight into water as you go to prevent them oxidizing and turning brown. Peel the yam and cut into slices, then rinse thoroughly in cold water to remove the starch.
Chop the yam, add to a large saucepan of lightly salted boiling water and cook for 10 minutes.
Meanwhile, peel the plantains and cut into chunks slightly larger than bite size. Add to the boiling yam at the 10-minute point and cook together for about a further 10 minutes until fork tender — they will continue to cook in the peanut sauce.
Strain, reserving the cooking water to use as vegetable stock for making the peanut sauce. Set the yam and plantain aside.
Prepare the peanut sauce: Heat the groundnut oil in a heavy-based saucepan, add the onion and sauté over a medium heat for 2 minutes. Stir in the chilli powder and curry powder, then add the garlic, ginger, Scotch Bonnet, crushed peanuts, sea salt, and black pepper and stir well – lots of punchy aroma should be rising from the pot at this point. Add the peanut butter 1 tablespoon at a time, while stirring, until it has all dissolved, then use a stick blender to blend all the ingredients to a smooth consistency.
Add the boiled yam and plantain to the sauce and leave to simmer for 20 minutes, stirring in a little water as necessary to prevent any sticking.
Serve in a bowl garnished with chopped red chillies and a touch of greenery such as sliced spring onions or puréed basil.