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Heat 2 tbsp of the oil in a medium saucepan. Add the onion and a pinch of salt and fry over a medium heat for 8 mins, or until the onions turn golden and sticky. Stir the flour and curry paste into the onions and cook for 2 mins. Remove from the heat and quickly whisk through the milk in several additions to make a thick sauce.
Bring a large pan of water to the boil, add the beans and cook for 1 min, then scoop out with a slotted spoon and set aside. Add the potatoes to the water and cook for 5-6 mins or until you can easily insert a cutlery knife into them. Drain and mash. Stir through the remaining oil, turmeric and nigella seeds, then season to taste.
Add the prawns and green beans to the sauce. Bring to a simmer and quickly remove from the heat. Stir through the coriander and crème fraîche and season to taste.
Heat the grill to its highest setting. Spoon the mixture into a medium heatproof dish. Spread over the mashed potato and grill for 4-5 mins or until golden and bubbling.
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