Boil potatoes, cool, peel and dice. Remove stems, wash, slit, deseed and chop green chillies. Scrape, wash and chop ginger.
Wash and chop tomatoes.
Clean, wash and chop coriander.
Pound red chillies with a pestle.
Cut buns into halves, horizontally.
Heat ghee on a large pan, add green chillies, ginger and tomatoes, saute over medium heat for 2-3 minutes, add red chillies and potatoes, reduce to low heat and with a metal spatula simultaneously mash and stir for 5 minutes.
While stirring keep sprinkling the ginger and garlic paste dissolved in 90 ml/ 6 tbsp of water.
Adjust the seasoning.
Increase to medium heat, add two-thirds of the butter and stir constantly until the butter is incorporated.
Sprinkle garam masala, coriander and lemon juice, and stir.