15g / ⅔ oz unsalted butter softened

50g / 2oz freshly grated parmesan

200ml / 7fl oz whole milk

1 tbsp cornflour

4 medium eggs

450ml / ¾ pt whipping cream

sea salt and black pepper

freshly grated nutmeg

200g / 7oz gruyère cut into 1cm / ½ in dice

● Have ready a 30cm / 12in oval gratin dish or equivalent dish, about 2 litre / 3½ pt capacity. 

● Bring a large pan of salted water to the boil, add the spinach, press down to submerge it and cook for 2 minutes, then drain in a colander and press out as much liquid as possible using the back of a ladle or a large spoon. Leave until cool enough to handle (or wear rubber gloves), squeeze it out even further with your hands, then coarsely chop it on a board using a large sharp knife. 

● Liberally grease the baking dish with the butter, then dust with half the parmesan. Blend a little of the milk with the cornflour in a large bowl, then break in the eggs and whisk. Whisk in the remaining milk and the cream, and season with salt, pepper and nutmeg. Stir in the spinach and the gruyère. Tip the mixture into the prepared dish. (You can prepare the recipe to this point several hours in advance, in which case cover and chill. If chilled, it may take a little longer to cook). 

● Preheat the oven to 200C/180C fan/gas 6. Scatter the remaining parmesan over the gratin and bake for 40-45 minutes until golden and puffy, and lightly set in the centre. Serve straight away.