125g flour, extra for dusting
½ teaspoon salt
75g unsalted butter, chilled and cut into small pieces
1 egg yolk
3-4 teaspoons cold milk
30g unsalted butter
4 shallots, finely diced
150g sorrel, stalks removed and roughly chopped
170g new potatoes, cooked and quartered
1 egg yolk
150g creme fraiche
salt and freshly ground pepper
170g gruyere cheese
For the pastry, pulse together the flour, butter and salt in a food processor until it resembles coarse breadcrumbs.
Tip the mixture into a bowl and add the egg yolk. Combine and add just enough milk to bring the dough together.
Turn out onto a lightly floured surface and pat together, flattening a little.
Wrap in plastic and chill for an hour.
Butter a sandwich tin, dust with flour and remove any excess. Roll out the pastry and line the tin, letting excess pastry hang over the sides. Chill for another 30 minutes.
Pre-heat the oven to 180°C. Prebake the pastry, weighing it down with rice on paper. Bake for 20 minutes, take from oven and remove the paper and rice.
For the filling, melt the butter over medium heat, sautee the shallots until soft, about 10 minutes. Add the sorrel and stir until wilted, stir in the potatoes, remove from heat and cool.
In a large bowl, whisk together the eggs, yolk and creme fraiche. Season well. Stir in the cooled sorrel and potatoes, and half the gruyere. Tip the mixture into the tart shell, smooth the surface and sprinkle on the remaining cheese.
Bake at 160°C for 30-35 minutes.