Put the flour, butter and pinch of salt into a food processor and pulse until it resembles breadcrumbs

Add the cold water, a tablespoon at a time, pulsing between each addition until the mixture comes together into a ball

Shape into a thick disc, wrap in clingfilm and refrigerate for half an hour

Melt the 30g of butter in a large, heavy-bottomed saucepan and gently cook the onion and garlic, with a little salt, until slightly golden, then add a good splash of water and cover

Sweat on a low heat until the onions are soft and sloppy - around half an hour - stirring occasionally

Preheat the oven to 180ºC/Gas mark 4 and put a metal baking sheet inside

Roll out the pastry and line a loose-bottomed tart tin 27cm diameter or 20 x 28cm (keep any leftover bits of pastry)

Chill for 20 minutes

Take the lid off the onions, turn up the heat and let the excess moisture bubble off and the onions cook until they are golden and sweet

Wash the spinach and cook in a covered pan with just the water left clinging to it, about 4 minutes over a medium heat, until wilted

Drain in a colander and leave to cool

Line the pastry case with parchment paper, fill with baking beans and blind-bake on the preheated tray for 15 minutes

Remove the paper and beans and cook for a further 7-10 minutes

Remove from the oven and use any leftover raw pastry to patch any cracks

Squeeze the excess moisture out of the spinach, chop roughly, season and add to the onions

Mix the eggs, yolks, cream and mustard and season well

Spread the onion and spinach in the tart case, sprinkle on the cheese and pour on the cream mixture

Put back into the oven, on the baking sheet, for 30 minutes, until set in the middle and golden

Allow to cool for 10 minutes before serving