To make the pastry, put the flour in a bowl, add the butter, and rub together with your fingertips until the mixture resembles breadcrumbs. Using a round-bladed knife in a cutting motion, combine the egg yolks with the mix until a pastry ball forms. Turn out on a lightly floured surface and quickly knead until the pastry is even and smooth, then wrap and place in the fridge to chill for 30 minutes.

Heat the oven to 200°C/gas 6.

Roll out the pastry on a lightly floured surface and line a 30cm push-up-bottom flan tin, making sure there are no cracks. Return to the fridge to chill for another 20 minutes, then line with baking parchment and fill with baking beans. Bake for 20 minutes, reduce the heat to 160°C/gas 3, and cook for a further 15 minutes, or until the pastry is golden and set.

While the pastry is baking, make the filling. Melt the butter with the oil in a saucepan. Add the finely sliced onions and cook over a low heat for 30 minutes. This long, slow method of cooking makes the onions melt; do not allow them to brown or frizzle.

Mix the cream, eggs, nutmeg and seasoning in a bowl. Put the cooked onions in the baked pastry case, carefully pour in the egg mixture and then return the tart to the oven to cook for 25 - 30 minutes. Serve warm or cold.