While the pastry is chilling, make the pasty filling. Heat the olive oil in a saucepan set over a medium heat, and add the chopped onions, the chopped garlic and the finely grated root ginger. Season with salt and freshly ground black pepper and cook until the onions are soft and slightly golden.
Grind the coriander seeds and cumin seeds, or the chopped thyme leaves, whichever you're using, and the black mustard seeds together using a mortar and pestle, then add the mixture to the pan, along with the minced lamb, the tomato puree, the English mustard and the Worcestershire sauce. Cook over a medium heat for 15 minutes or until the lamb is cooked. Add the fresh or frozen peas, whichever you're using, for the last 1-2 minutes of cooking. Add the chopped fresh mint, then season to taste with more salt and freshly ground black pepper and allow to cool.
Roll the pastry out on a lightly floured work surface until it is approximately 2mm (1/16 of an inch) thick. Using a small saucer or something similar, cut the pastry into 10cm (4½ in) diameter circles.
Lay 1 generous tablespoon of the lamb mixture on one half of the circle and brush the edge of the other half with beaten egg, then fold the pastry over to form a semi-circle. Pinch the edges together to seal, making sure there is no air trapped inside, and mark the edges with a fork. Repeat until all the pastry circles and lamb filling are used up.
Brush the tops of the pasties with beaten egg and bake them in the oven for 15-20 minutes or until they are golden and a skewer inserted in the middle of each one comes out hot. Serve hot or at room temperature.