I usually make this pie to use up left over potato after making another Indian dish like Aloo paratha or Aloo masala.
Makes two small pot pies.
Start by making the pasty. Combine the water and vinegar in a cup and put in the freezer to chill. Measure out the flour and margarine and put in a food processor with the salt. Pulse until the mixture resembles coarse breadcrumbs. Pour in the water and vinegar slowly and blend until it comes together as a pastry.
Tip out the pastry onto the bench and press together into a ball, wrap in cling wrap and put into the fridge until it is needed.
Cut the potato into small chunks, add to saucepan of water and boil until soft. Cut the carrot and cauliflower into small pieces and add them with the peas to the potatoes when they are almost done. Drain the vegetables and set aside.
Finely slice the onion and saute it in a fry pan with a little oil. Cook until soft and then add the cumin, coriander, mustard seeds and chilli. Roughly chop the tomato and add it to the onions. Cook for another two minutes and then add the cooked vegetables and few tablespoons of water. Stir through and set aside.
Turn on the oven to preheat to 180C / 360F.
Divide the pasty into three. Roll out two of the pieces into large enough discs to fill your dishes. Fill each dish with half the vegetable mixture. Divide the remaining piece of dough in half. Roll each piece into discs to fit the top of the pies. Press the tops firmly onto the bases with a form. Slice some air holes into the top of the pies.
Bake the pies in the oven for 30 minutes. Remove them from the oven and brush the tops with a little oil. Return to the oven for ten minutes. Remove and serve warm.