Heat oil in a deep bottomed pan, and splutter mustard seeds. Throw in sliced onions and fry until they turn soft and dark brown in color. Fry garlic and ginger pastes. Now you can add the spices– ground turmeric, ground coriander, black pepper, and garam masala – and fry them in hot oil. Add in the capsicum slices and give it a good stir. Add enough water just to cover the capsicum and the spices. Cover, and simmer over low flame. When the oil separates, add chicken pieces along with enough salt. Add more water if needed. Throw in curry leaves. Cover, and cook again over medium flame. When capsicum and chicken are cooked well, remove curry from flame. Serve hot.