Peel and chop the large waxy potatoes into chunks. Add to a large pan and cover with cold water. Bring to the boil and cook until softened. Drain the potatoes, roughly mash and keep to one side until required Heat the vegetable oil in a saucepan over medium heat. Add the onions and green chilli and fry until translucent. Next, stir in the ginger-garlic paste and ground turmeric and fry for 2 more minutes. Add ¾ of the chopped tomatoes and the tomato paste. Cook and stir for 3-4 minutes until the tomatoes start to break down. Stir in the carrots, green pepper, petit pois and cannellini beans and a pinch of salt. Cook for a further 5 minutes, stirring well. Add the mashed potatoes and the rest of the tomato to the pan, then add 250ml (8fl oz) of hot water. Allow to simmer for 8-10 minutes to reach a thick, stew-like consistency. Take the pan off the heat and sprinkle in the garam masala, lemon juice and chopped fresh coriander. Stir well and add butter if you wish. Preheat the grill to a high heat and toast the baps for 1-2 minutes on each side. Divide the vegetables onto plates and serve alongside the toasted baps.