Heat a deep, heavy frying pan or flameproof casserole dish over a medium heat. Tip in the butter and olive oil and, once the butter has melted, stir in the leeks and thyme. Stir in a pinch of salt and cook for 15-20 mins until the leeks have softened. Add the garlic and cook for another minute until fragrant before adding the ham and flour. Stir well, ensuring everything is coated in the flour, then splash in a little of the sherry.

Mix well and gradually pour in the rest of the sherry, stirring after each addition. Pour in the stock. Bring to a simmer and bubble for 5-10 mins until thickened. Stir in the herbs, cream and English mustard, then season to taste. Tip into a lipped pie dish and leave to cool. Discard the thyme sprigs.

Mix the wholegrain mustard and egg together. Brush the rim of the pie dish with a little beaten egg, then drape over the pastry. Trim the edges, then press with a fork to seal or use your fingers to crimp the edges. Use any pastry offcuts to decorate the top. Brush the pie with the remaining egg mixture and sprinkle with sea salt flakes. Chill until ready to bake, and brush with more glaze before baking. Will keep chilled for up to 24 hrs or frozen for up to three months.

Heat the oven to 220C/200C fan/gas 7. Bake the pie for 25-30 mins until deeply golden. Serve with buttered new potatoes and peas.