To make the pastry, put all the ingredients (except the water and the egg for brushing) in a bowl and rub them together with your fingertips until the mixture resembles breadcrumbs.
Add 5 tbsp water one tablespoon at a time and knead until it comes together and forms a ball. Wrap in cling wrap and chill for at least 30 minutes or until needed.
Lightly grease a 9 or 10-inch tart dish
Roll the dough out on to a lightly floured surface to about 1/8”thickness. Use this to line the buttered tart dish. Leave some excess pastry hanging over the rim as the pastry will shrink in the oven.
Prick the bottom of the dish with a fork before putting it in the fridge to chill for around 30 minutes.
Preheat the oven to 325 F
To make the filling, place all ingredients in a mixing bowl with 2 pinches of salt and whisk.
When ready to bake, remove the dish from the fridge, line the pastry with greaseproof paper and fill with baking beans. Bake until the edges are golden (about 25 minutes).
Now remove the greaseproof paper and baking beans, brush the dough with 1 beaten egg and bake again for 5 minutes or until the bottom looks cooked.
Remove the dish from the oven and turn the temperature down to 300 F
Pour ¾ of the filling into the tart dish immediately, place the tart back in the oven and carefully top it up with the remaining filling(this is to prevent it from spilling out when filled to the top when you put it back into the oven).
Bake for about 60 minutes or until, when you shake the tart lightly, the middle doesn’t wobble. Let cool for 15 minutes, then trim the edge of the pastry with a sharp knife.
Cut into 6-8 pieces, serve with spicy greens, sliced duck prosciutto and a thick Dijon vinaigrette.