Cut the plantains up into 6 pieces each. Bring the water to a boil and add the plantains. Cook for 10 minutes, then add the tomatoes, pepper and onion.
Simmer for 10 more minutes, then add the stock cube. Cover the pot and give the cube about five minutes to dissolve, then add the palm oil.
Cook for an additional 10 minutes and season to taste with salt and pepper. Sprinkle with cashews and serve.