In a large frying pan or saucepan, heat the oil over medium heat. Add the onion, and cook, stirring frequently until it has softened. Sprinkle in the ginger, garlic, coriander, cardamom, nutmeg and cumin. Cook for 3 minutes more, stirring frequently.
Pour the rice into the saucepan and sauté the rice with the spices for 2 minutes, stirring constantly. Pour the lentils and salt into the saucepan. Pour water into the pan, stir.
Place the potato into the pan. Bring the mixture to the boil, cover and turn heat to low. Cook for 10 minutes.
Place the red capsicum, peas and sultanas into the saucepan. Stir well, then cover the pan again. Cook 10 minutes more, or until the rice, potatoes and lentils are tender. Stir in the butter, if using. Serve and enjoy.