Preparation method

1. For the pastry, sieve the flour into a mixing bowl and add the salt. Add the butter and, using your fingers, gently rub the butter into the flour until it resembles breadcrumbs. Add the beaten egg and cold water, and gently bind together until you can press the dough into a ball.
2. Wrap in cling film and refrigerate for at least two hours before using. You will use half of the shortcrust pastry for this recipe – the remainder can be refrigerated or frozen and used for another recipe.
3. For the béchemel sauce, stud the onion with the clove. Pour the milk into a saucepan and add the studded onion, garlic, bay leaf, thyme and grated nutmeg.
4. Bring to the boil over a high heat, then immediately turn the heat down to a simmer. In another pan, melt the butter over a moderate heat and, when melted, stir in the flour. When thoroughly mixed and just as it begins to take on a little colour, gradually add the milk, straining it through a fine conical metal strainer, stirring with each addition. Discard the onion, garlic and thyme.
5. Cook the sauce for about 30 minutes over a low heat stirring regularly. The sauce should be smooth and lump-free. Pass the finished sauce through a conical strainer into a bowl. To stop the sauce from forming a skin, place one or two small pieces of butter on top of the sauce while it is still hot.
6. Preheat the oven to 180C/350F/Gas 4. Fort he tart, roll out the pastry and line a 28cm/11in loose-bottom tart tin, or alternatively four individual tart tins. Line the pastry with aluminium foil and fill with ceramic baking beans. Bake blind for 15 minutes, then remove the foil and baking beans and bake for a further 3-5 minutes, or until the pastry is slightly coloured. Remove from the oven and allow to cool.
7. Heat a frying pan on a high heat, add the olive oil and, as soon as it starts to smoke, add the onions and garlic and fry until golden-brown (this will take about 10 minutes). Add the thyme and bay leaf. Pour in the wine and boil until it has reduced to half the volume.
8. Stir in 90g of the béchemel sauce and continue to cook until it has been completely absorbed into the mixture. The remainder of the sauce can be set aside for another dish.
9. Remove the pan from the heat and stir in the parsley and egg yolk – this will give the mixture richness and help to glaze the tart. Season with a little salt and freshly ground black pepper. Preheat the grill to hot.
10. Pour the onion mixture into the pastry case and sprinkle the top with the grated cheese. Brown the tart on a tray under the grill until crisp and golden-brown. Serve with a green salad.

    With added bacon, in this case.