In a food processor, use the dough blade to mix the flour and salt, add the butter and blitz until you have a breadcrumb consistency. Do not over mix. Remove from the processor and put in a mixing bowl. Slowly pour the water in, a tablespoon at a time, and mix until the dough comes together.
While the pastry is still cool, dust a surface with flour and roll out on a lightly floured surface to 1/4cm inch in thickness. Line a 25cm x 4cm tart ring with the pastry. When you have lined the tin, press the edges in well. Taking the rolling pin, run it across the top of the tin to trim all the excess pastry. Now press your fingers on the inside of the edge so the pastry is just peeking over the tin. Place this in the fridge for 20 minutes to firm up, this will ensure that the pastry doesn't shrink during cooking. Pre-heat the oven to 200°C. Meanwhile slice the leeks into 1/2cm rounds. In a saucepan on a medium heat, melt the butter and sweat off the leeks for five minutes without color. Add the thyme and allow to cool.
Remove the cooled tart case from the fridge. Line the pastry with greaseproof paper and fill with baking beans to keep the base's shape, and blind bake at 200°C for about 20 minutes. Remove the baking beans and greaseproof paper and return to the oven for a further 5-8 minutes or until the pastry is starting to turn golden brown. Remove from the oven, brush with egg yolk and allow to cool.
Cut the imbolc, or alternative semi-hard cheese into 1cm cubes and sprinkle into the pastry tin. Cover with sliced leeks. Grate the lammas with nettle cheese and sprinkle over the leeks.
Whisk together the eggs and yolks and then whisk in the cream and milk. Season with salt and plenty of fresh crackled black pepper. Pour into the pastry case and lastly crumble over the lammas cheese and bake in a 180°C oven on the middle shelf for 30 minutes or until set.
Allow to cool for an hour and serve with the tomato and onion salad.
For the tomato and sharp red onion salad with maple dressing, slice the onions and beef tomatoes, layer on a large plate. Shake all the vinaigrette ingredients together in an old jam jar until thickened and pour over the onions and tomatoes.