Pie Crust: In a food processor, blend the flour and salt. Add the butter and process in short bursts until the the mixture is crumbly. Should take only a few bursts to make the pieces of butter a little smaller than pea-sized. Add in chilled water then quickly process. Add more water if needed, tablespoon by tablespoon, until the mixture starts to form a soft dough. Form into a ball and wrap in plastic wrap. Place in the fridge for 1 hour.
Filling: Fill a large saucepan with water and bring to the boil. Add potatoes and cook until just tender. Drain and set aside.
Melt the butter in a large frying pan. Add leeks, cabbage and mushrooms then cook until soft. Stir in garlic and thyme then cook for a few more minutes. Mix in stock and enough water to make a thickish sauce. Stir in potatoes and remove from heat. Spoon mixture equally into two pie dishes.
To Assemble: Roll out 1/2 of the pastry on a floured surface to form a rough circle about 3mm thick. Cover one pie dish with the pastry, crimp edges and make several small cuts in the top to allow steam to escape while cooking. Repeat with remaining ingredients.