Cook the potatoes in a pan of boiling water for about 20 minutes, or until soft, then drain and allow to cool.
When the potatoes are cool, peel off their skins, return to the pan and mash.
Put the garlic and ginger pastes in separate bowls and stir in 125ml / 4 1/2fl oz (1/2 cup) water. Set aside.
Heat the ghee or oil in a heavy-based frying pan (skillet), add the tomatoes, onions, chillies, ginger and turmeric and stir-fry over medium heat for about 4-5 minutes.
Reduce the heat, add the mashed potatoes, chilli powder and salt, mix thoroughly and continue to cook, mashing and stirring for about 5 minutes.
Add the garlic and ginger paste mixtures, then increase the heat, add 4 tablespoons butter and mix well.
Sprinkle the garam masala, chopped coriander and lemon juice over and stir well.
Slice the pav in half.
Heat the remaining butter in a pan and, when melted, add the pav halves and fry until light brown.
Remove from the pan and serve with the mashed vegetables.