(The Complete Vegetarian Cuisine)
Put flour in a mixing bowl with the salt and margarine. Using your fingertips, rub the fat into the flour until the mixture resembles fine breadcrumbs. Add the water and gently press the mixture together to form a dough.
Turn the dough onto a lightly floured board and knead lightly. If you won't be using it right away, wrap in plastic and store in the refrigerator for up to 24 hours. Otherwise, press it into the general shape required by the recipe and roll it flat with a rolling pin.
Sauté the onion in oil for 5 minutes, then add the diced potatoes and garlic. Cook gently for 10 minutes. (The cookbook says 10 minutes, but in Gene's experience it's more like 15 to 20, and you have to cook at lower heat or garlic will burn and taste bitter.) Add the mushrooms and cook for another 4-5 minutes, until the vegetables are just tender. Add the parsley, season to taste with salt and black pepper. Turn off the heat and let the filling cool.
Preheat the oven to 400°. Divide the pastry into 4 equal pieces. Roll each piece into a circle 6 inches in diameter. Spoon a quarter of the potato mixture into the center of each circle. Fold over the pastry and press the edges together.
Make a couple of steamholes in each turnover. If you'd like brownish gloss to your turnovers you can glaze them with a beaten egg. Place turnovers on a baking sheet and bake for 20-25 minutes until golden brown. Egg-glazed turnovers will look more golden-brown than non-glazed but there is no difference in taste.