For the filling, peel and slice the onions as finely as possible, taking care to discard the fibrous parts at the root of the onions.
Melt 60g/2oz of butter and a little oil in a heavy frying pan. In this cook the onions, covered, until they are quite soft and pale golden. They must not fry, and they should be stirred from time to time to make sure they are not sticking. They will take about ½ an hour. Season with salt, nutmeg and pepper.
Stir in the very well beaten yolks and the cream, and leave until the time comes to cook the tart.
Oil a 20-cm/8-inch tart or flan tin. Roll out your crust as thinly as possible (the great thing about this dish, as also the quiches of Lorraine, is that there should be a lot of creamy filling on very little pastry). Line the tin with the pastry, pressing it gently into position with your knuckle. Pour in the filling, cook in the centre of a fairly hot oven, with the tin standing on a baking sheet, at 400F/gas 6, for 30 minutes. Serve very hot.
Recipe from Elizabeth David's At Elizabeth David's Table
https://theviewfromgreatisland.com/elizabeth-davids-onion-tart-gourmets-50-women-game-changers/ September 9, 2011