Boil the potato in its jacket. Dice finely and set aside. Chop onion and tomato, finely mince the ginger. Fry onion in a little oil till golden, add tomato and ginger and fry for 1-2 minutes. Add the garam masala, chili powder, sugar, peas and cook till peas are tender. Add water if necessary. Stir in the chopped potato and fry for five minutes. Remove from the heat and mix in the raisins, coriander leaves, lemon juice and salt. For the pastry, sieve the flour, salt and baking powder into a bowl and mix to a firm dough with the butter, milk and a little water if necessary. Knead well and roll out thinly into circles. Cut each in half put a spoonful of the stuffing on each, damp the edges of the pastry and fold to form a triangle. Seal edges well and deep fry in oil till golden. Serve hot.