Make the flour into a ''well'' on the pastry board, beat the egg and add it, with the butter, salt and a little of the water, to the centre of the well, then work the ingredients into the flour with your fingertips, incorporating more water little by little; whether you have to use all the water or not depends on the quality of the flour.
Press all the dough together into a ball then, finally, work it thoroughly with the heel of your hand, gathering it up with pastry scraper as you do so until it is soft and pliable. Leave it in the refrigerator to firm up again before you roll it out.
Peel the onions. Cut them into fine rings and weigh out 300 g.
Cook the bacon strips (which need to be VERY finely cut) in the butter. Let them take on a golden colour, then add the sliced onions and leave them to cook gently until they are tender.
In a bowl beat together the cream, milk and eggs, adding salt, pepper and nutmeg to taste.
When the onions are cooked, add them to the mixture in the bowl and add the chopped parsley.
Roll out the dough 2mm thick and use it to line a buttered tart tin 26 cm in diameter and at least 3cm deep. Cut a disc of aluminium foil of the same diameter as the tin and place it on the pastry. Weight it down with a handful of beans or crusts.
Pre-heat the oven to 150 oC (300 oF) and cook the pastry ''blind'' for half an hour. Watch the coulouring of the edge and as soon as it starts to turn golden remove the aluminium foil and beans or crusts. Cook the pastry shell until the base is baked but not coloured.
Fill the shell with the onion mixture. Put the tart back into the oven. Check that the temperature is still 150 oC and keep the oven at or just below this temperature for about 1 hour. Towards the end you could reduce the heat a little, as the tart needs TO COOK VERY SLOWLY. The tart is ready if, when you shake it, the filling appears to be quite firm with no hint of a whole in the centre.
Serve warm, in small slices !