You will need a 27-centimetre tart tin with removable base. In a food processor, mix the butter and flour and when it has formed fine crumbs, add enough water until it forms a ball. Wrap the pastry in cling film and chill it for an hour while you get started on the filling.

Melt the butter in a heavy-based saucepan with a snug lid and sweat the onions, ever so slowly, for at least 25 minutes. They’ll need the odd stir and they should shrink down by at least half.

Try not to colour them. The steam that gets trapped in the saucepan and the low heat should help keep them sweating rather than sautéing. Season really well, then remove from the heat and allow them to cool down.

Preheat the oven to 180 degrees/gas 4. Roll out your pastry between two sheets of parchment paper. You will not have a lot of excess pastry. Line the tart case, and then cover with scrunched up parchment/baking paper and fill with dried beans and bake for about 25 minutes. Carefully remove the paper and beans and then cook for another few minutes to dry out the tart shell.

By this stage, the onions should be cool. Add the cream, Dijon mustard and thyme and stir well. Then add the eggs and egg yolks. Season lightly and mix in about a third of the Gruyère.

Put the tart case on a baking tray and fill with the onions at first, to distribute evenly and then ladle or spoon in the liquid. Don’t overfill as it will leak all over the oven. Sprinkle the Gruyère on top and bake for 20 minutes, by which stage you may be able to top up with a little more filling, if you haven’t been able to fit it all in.

Bake until just set, which is about 35-40 minutes in total. Let it cool down and rest before serving. It will keep on cooking. This is best served while still warm or at room temperature.