Finely chop one of the lime quarters, including skin. Heat 2 tbsp oil in a medium saucepan then fry onions over a medium heat for 5 mins until softened and starting to colour. Add the curry powder and tomato purée then fry for another min. Tip in 200ml water, lentils, mango chutney and the chopped lime then bring to the boil. Simmer for 5 mins or until thickened.
Heat remaining oil in a frying pan. Season the fish with salt and pepper to taste then fry for 5 mins, turning halfway, until golden and cooked through. Add a squeeze of lime to the pan and to the lentils then serve together with naan bread or chapatis on the side.