Scrub the potatoes and sweet potatoes and chop into 2cm chunks, cook in a large pan of boiling salted water for 10 minutes, or until tender, then drain and steam dry. Peel the garlic and ginger, finely chop with the chilli, and place in a large non-stick frying pan over a medium heat with the butter, garam masala and mustard seeds. After 1 minute, tip and mash in the potatoes, then season to perfection with sea salt and black pepper. Keep frying until crispy, then mix up and allow to get crispy again. Divide roughly into 4 (still in the pan), then use 2 spoons to crudely mould and shape into balls, patiently frying and turning until kind of rounded, really golden and crispy all over (trust me, these are amazing!).
Spoon the mango chutney into a bowl, squeeze in enough pomegranate juice to loosen, then mix together with a handful of the pomegranate seeds. Whiz the mint leaves in a blender with the yoghurt until smooth. Split the rolls open and lightly toast on the inside, and roughly crush the Bombay mix.
Spoon a dollop of mint yoghurt on to each bun base, top with a hot potato ball, a little mango chutney and Bombay mix, then pop the lid on and squash.
As another option, these are also great torn into a chapatti with a mixed chopped salad and a squeeze of lemon juice for an Indian-style wrap.