Sift the flour into a large bowl and add the chopped fresh thyme, salt and pepper.
Add the chilled butter in pieces and stir through with the blade of a butter knife. Rub the fat into the flour mixture with your fingertips until combined. You should end up with a mixture that looks like breadcrumbs.
Add ice-cold water very gradually, stirring it through with a fork, until lumps begin to form. Bring it all together quickly into a ball with your hands. Flatten into a disc and chill for half an hour. (You may not need all the water, you may need a little more.)
While the pastry chills make your filling.
Lightly grease the flan dish.
On a lightly floured surface roll the pastry out to fit your flan dish.
Drape the pastry over the flan dish and persuade it into place using a small ball of pastry to press it into the edges of your dish.
Chill for a further 20 minutes.
Prick it lightly all over with a fork, line it with foil or baking parchment and fill with baking beans. Bake blind at 200C/390F for 15 minutes. Remove the beans and parchment and bake for a further 8 minutes.
For the Creamy Leek Filling
Wash the leeks well and slice.
Melt the butter gently in a large saucepan, add the chopped leeks and raise the temperature until they just begin to sizzle.
Turn the heat to low and put the lid on. Cook very gently for about 30 minutes until the leeks are very soft. Try not to remove the lid as they steam, shake the pan if you are concerned that they may be sticking.
Blend or whisk the silken tofu until smooth.
Mix in the grated cheese, thyme and mustard and season well with salt and pepper - taste and adjust seasonings and flavours accordingly.
Stir the leeks into your creamy, cheese mixture and spoon into the prepared tart shell. Sprinkle with a little grated nutmeg and bake in a preheated oven at 180C/350F for 30 minutes until set.
Serve warm or cold.
Notes
Nutritional information is only an approximate guideline. Calculations will vary according to the ingredients you use and your cooking methods.