Chilling time: 15 minutes / Cook time: 1 ½ hours, plus resting 1. Preheat the oven to 200C/180C fan/Gas 6 and grease the tin with butter. 2. To make the pastry, measure the flour and butter into a food processor and whizz until the mixture resembles breadcrumbs. Add the beaten egg and water and whizz into a smooth ball of dough. Alternatively, rub the flour and butter together in a mixing bowl with your fingertips before adding the egg and water. 3. Roll the pastry out on a lightly floured work surface to a disc slightly larger than the tin and about the thickness of a £1 coin. Carefully transfer the pastry to the tin and press it into the base and sides. Form a little lip around the edges of the tin, trim off any excess and prick the base with a fork. Place in the fridge to chill for about 15 minutes. 4. Line the pastry case with baking paper and baking beans and bake blind in the oven for 15 minutes. Remove the beans and paper and return to the oven for a further 5–10 minutes or until just cooked and pale golden. Reduce the oven temperature to 180C/160C fan/Gas 4. 5. Meanwhile, make the filling. Heat the oil in a wide-based frying pan, add the leeks and celery and fry over a high heat for 2–3 minutes. Reduce the heat to low, cover with a lid and simmer for 20–25 minutes or until the vegetables are completely tender. Take off the lid, then increase the heat and fry for another minute to drive off any excess liquid. Remove the pan from the heat. 6. Beat the eggs and cream together in a jug and season with salt and pepper. 7. Spoon the cooked leeks and celery into the pastry case, scatter with the cheese and parsley and season with salt and pepper. Pour over the egg and cream mixture. 8. Bake in the oven for about 35–40 minutes or until golden brown and just set on top (see tip). Leave for 5–10 minutes to rest before turning out. Prepare Ahead: Can be made up to 8 hours ahead and reheated. The pastry base can be cooked up to a day ahead. Freeze: Both the cooked tart and the unfilled pastry base freeze well. Mary’s Classic Tip: * Check the tart halfway through cooking and if it is browning on one side more than the other, turn it around on the oven shelf.