Riverford Kale, Celeriac and Mushroom Pie

method:

• Turn the oven to 190C/375F/Gas 5. Heat three tablespoons of oil in a pan. Add the celeriac – it will hiss and spit for a few seconds – and keep it moving so it doesn't catch or burn. After a few minutes, add 50ml water, half the butter, one tablespoon of white wine vinegar and a good pinch of salt. Give it a good stir, cover with a tight-fitting lid and drop to a medium heat. Keep checking every five minutes, adding a dash more water if it looks like it may be catching on the bottom. You want it to cook through so the edges become soft and the middle retains a toothsome bite; this should take about 20 minutes. Remove the lid for the last five minutes to evaporate any excess liquid, then season to taste.
• Meanwhile, melt the remaining butter in a frying pan with the remaining tablespoon of oil, and fry the onion on a medium heat until starting to soften; about 10 minutes. Add the sliced mushrooms and cook until they start to release their moisture.
• Pour in the wine, add the chopped garlic and let the liquid bubble and reduce until you have a dark, sticky mess. Season with salt and pepper.
• While the mushrooms cook, blanch the kale in salted water in two batches for 1½ minutes each. Remove with a slotted spoon and plunge straight into ice cold water to stop the cooking and fix the colour. When cool, squeeze out the water and roughly chop.
• Combine all the pie components in a large bowl. Pour in the cream, grate in the nutmeg, mix gently and check the seasoning. Pack the mixture snugly into a shallow pie dish. Cut out the pastry to the size of the rim of your dish and place it over the top of the filling, tucking it around the sides. Brush with the egg and slash a couple of holes in it for the steam to escape. Bake for 20–25 minutes, until the pastry is golden and the filling is bubbling around the edges.