To make the pastry, add the flour, butter and salt to a large mixing bowl. Rub the butter in with your fingertips until you have rough breadcrumbs.
Add the egg yolk and cold water and bring the dough together with one hand. Add more water as needed, just 1 tablespoon at a time.
Shape the pastry into a disc, wrap it in baking paper and chill in the refrigerator for at least 30 minutes, but ideally 2-4 hours.
Preheat the oven to 180°C fan (400°F/gas 6). Grease a 24cm (9.5in) tart tin (pan) with butter. Place the pastry on a lightly floured work surface and dust the dough with a little flour.
Roll out to 1-2cm (½-¾in) larger than the tart tin. Move the pastry frequently and dust with extra flour, if needed. Line the tin with the pastry. Chill for 10 minutes.
Meanwhile, prepare the filling. Melt the butter in a large saucepan over a medium heat. When it begins to foam, add the spinach and cook until it has wilted completely.
Remove from the heat and spread over a tray to cool, then transfer it to a sieve (strainer) to drain for 10 minutes. Place the tart tin onto a baking sheet, cover with baking paper and fill with baking beans. Blind bake for 12-15 minutes, or until the edges of the pastry are golden brown.
To finish the filling, add the milk, cream and mascarpone to a mixing bowl and whisk together until well combined. Add the eggs and yolks, season with salt and pepper and whisk together.
Add the grated parmesan and combine, then add the drained spinach and fold together. Remove the tart from the oven and remove the baking paper and beans. Reduce the oven temperature to 160ºC fan (350ºF/gas 4).
Pour the filling into the tart shell, carefully return the tart to the oven and bake for a further 35-40 minutes until slightly golden and crisp on top.
Allow to cool, then cover and refrigerate to firm up before serving.