Heat the oven to 200C/180C fan/gas 6. Heat the oil in a frying pan or large saucepan over a medium heat and fry the leeks with a pinch of salt for 5-6 mins until softened and beginning to brown. Add the kale and cook for a few minutes until wilted. Remove and set aside.

Either in the same pan or a medium saucepan, add the butter and, once melted, stir in the flour to make a thick paste. Stir in the double cream and mix until smooth, then pour in the milk (you can whisk it if it’s a little lumpy). Once you have a smooth sauce, leave to simmer for a couple of minutes, then remove from the heat and stir in both cheeses until melted.

Mix the leeks and kale back into the cheese sauce, then tip into an ovenproof dish (we used a round 23cm dish). Bake in the oven for 25-30 mins until simmering and golden on top.