Combine the pot barley and water in a small pan and bring to a boil. Cover and simmer for 45 minutes, or until all the water is absorbed.
Preheat the oven to 220°C/ 200°C (fan)/ 425°F/ Gas 7. Grease a 26 cm oval casserole pan with olive oil and set aside. You could also use an 8x10 inch casserole pan.
Clean the leeks, top and tail and chop into thin rounds. Chop the mushrooms roughly, strip the thyme leaves from the stems and mince the garlic.
Heat oil in a large skillet over medium heat. Add the mushrooms and saute for 3-4 minutes.
Add the leek, thyme leaves and garlic and season with salt and pepper. Continue cooking over low heat for a further 3 minutes.
While the mushroom and leeks are cooking mix the flour with the vegetable stock and set aside 1 tablespoon of the single soya in a small dish.
Add the single soya, mustard and vegetable stock (with flour) and simmer until the mixture starts to thicken a bit, about 5-6 minutes.
Stir in the barley, and pour everything into the prepared pan.
Unroll one sheet of puff pastry on the top of the pan, and roughly crimp any bits that are hanging over the edges. Brush the reserved single soya over the top of the pie and bake for 20-25 minutes, or until the crust is puffed a golden. Enjoy!!