For the Pastry
100g Shipton Mill organic oat flour
100g Shipton Mill organic buckwheat flour
1 tsp Xanthan Gum
Pinch of salt
100g Cold butter, cubed
1 egg, beaten and cold water to bind
For the Filling
1 large leek, finely sliced
2 cloves garlic, crushed
A few chestnut mushroons, sliced
Fresh tarragon, chopped
4 large free-range eggs, beaten
284 ml carton double cream
150g mature Cheddar, grated
1/2 tsp English mustard powder
Preheat oven to 200C/400F/Gas 6. Grease an 8 inch tart tin.
Sift the flours, Xanthan and salt into a bowl and mix. Add the butter and rub it in with your fingertips until the mixture resembles fine breadcrumbs. Alternatively, pulse the ingredients in a food processor.
Add the egg and mix it in with a palette knife or round-bladed table knife. Gradually add 1-2 tbsp cold water, a few drops at a time and mixing it in after each addition. Keep adding water and mixing until it comes together to form a dough.
Transfer the dough to a floured board and knead briefly and gently until smooth. Wrap in cling film and chill in the refrigerator for approx 30 minutes.
Whilst the dough is resting, prepare the filling. First, sweat the leeks and crushed garlic in a frying pan with a generous knob of butter until soft but not browned. To effect this, add a good pinch of salt to the leeks and place a round of tin foil on top of the leeks in the frying pan. Set aside.
Meanwhile, in a separate pan, melt a generous knob of butter and add the sliced mushrooms and fry gently, adding the chopped tarragon. The mushrooms need to be soft but not brown. Set aside.
Break four eggs into a bowl and beat well and add the cream, incorporating it into the egg mixture. Season with freshly ground black pepper and the mustard powder and mix in thoroughly.
Grate the cheese and set aside.
Remove the pastry from the cling film and roll out on a lightly floured board to about 5mm depth (1/4 in) thick and large enough to fill the tart tin.
Carefully wrap the pastry around the rolling pin, lift over the tin and unroll the pastry. Press the pastry gently into the base and sides of the tin, pressing it into the flutes if your tin is fluted. Trim the edges with a knife, repair any holes and prick the pastry base with a fork.
Line the pastry with baking parchment and weigh down the parchment with ceramic beans (or you can use ordinary dried beans, such as haricot beans, or rice). Place on a baking sheet and bake in the preheated oven for 10-15 minutes. The pastry needs to remain quite pale.
Remove the tart from the oven and carefully lift out the parchment and beans. Return to the oven for another 5 minutes to crisp up and remove from the oven. Reduce oven temperature to 180C/Gas 4.
Spoon the mushrooms and leeks into the pastry case and pour in the egg and cream mixture. Sprinkle on the grated cheese.
Return the tart to the oven and bake on the middle shelf for about 30-35 minutes until the mixture is golden, set and slightly puffed up. Remove from the oven and to cool a little before removing from the tin. Serve warm or at room temperature, with vegetables or a mixed salad.