Make the pastry by sifting the flour and mustard powder into a mixing bowl, then stir in 40 mls of oil. Add cold water one spoonful at a time until you have a dough that forms a ball and leaves the bowl clean. Leave to rest at room temperature for half an hour.

Meanwhile, prep the filling. Put the oil in a heavy-based saucepan, then add the onions, stir to get them well coated in the oil, and cook them (uncovered) over a medium heat for about half an hour until they have reduced and turned a deep brown (caramelised).

If you cook them for less long, or like them less caramelised, you'll need to cook 50-100g less onions, as they will have reduced less in size.

Roll out the pastry to the thickness of a 20p piece to line the tart tin, prick the base with a fork, place it on the pre-heated baking sheet, and bake in the centre of oven for up to 15 minutes. Check at 5 minutes and if the pastry seems to have puffed a little and turned a strawlike colour (ie barely browned), remove earlier than the 15 mins. Then spread the onions all over the base of the tart, whisk the beaten eggs together with the milk and some seasoning, and pour this mixture over the onions.

Finally sprinkle both cheese over the top, return the tart to the oven and bake for 30 minutes or until the filling is puffy and golden brown.

Serve with 320g peas and 320g carrots (80g per person).