Method
First make the pastry. Pulse the flour, butter and salt in a food processor until the mixture resembles breadcrumbs. With the motor running, add the water gradually, stopping as soon as the dough comes together. Remove the dough, knead briefly, wrap in cling film and chill for at least 30 minutes.
Heat the oven to 180°C fan. Roll out the pastry to line the tart tin, leaving some overhang. Prick the base, line with baking parchment and baking beans and bake for about 20 minutes. Remove the parchment and beans and bake for a further 5 minutes, until dry and lightly coloured. Trim the edges and set aside.
Trim away most of the darker green leek tops. Cut the remaining leeks into 2cm rounds and wash carefully. Heat a large sauté pan over a medium heat and add the butter and pancetta. Fry until the pancetta begins to crisp, about 4 to 5 minutes.
Add the leeks, a couple of tablespoons of water, half the thyme, the lemon zest and some seasoning. Stir well, cover and cook for 6 to 8 minutes, until just tender. Squeeze over the lemon juice and remove from the heat.
Allow the mixture to cool slightly. Whisk the eggs, yolks, cream and 100g ricotta together and season.
Arrange the leek mixture evenly in the tart case and pour over the custard. Lay the lemon slices on top and dot with the remaining ricotta and thyme.
Bake for about 35 minutes, until set and lightly blistered. Leave to cool for at least 25 minutes before serving.
