Heat the vegetable oil in a pan and add the black mustard seeds and curry leaves, if using. Cook for 2-3 minutes or until popping and fragrant, then add the fennel and a pinch of salt. Cook for 5 minutes or until starting to soften, then spoon in the curry, ginger and garlic pastes and tomato purée. Cook gently for 5 minutes. Tip in the coconut milk, bring to the boil and simmer for 10 minutes. Add the cherry tomatoes and green beans, simmer for 2 minutes and then add the haddock with some seasoning and simmer gently for 5 minutes or until the haddock is cooked through. Finish with the lime juice and serve with rice.