To make the chutney, add all of the "chutney ingredients" to a blender and blitz into a smooth sauce. Empty into a bowl and keep aside.
Now mash the potatoes and spread them on a plate. Sprinkle over the salt, turmeric, chilli powder & mango powder and mix well. Now heat 2tbsp of oil in a non stick pan. When hot, add the mustard seeds. When they sizzle and pop, add the coriander, ginger & garlic. Mix and cook for a a minute, then add the potatoes. Cook on a low heat for 2 minutes. Turn off heat and transfer to a plate and allow to cool.
To assemble the sandwich, butter one side of both slices of bread. Flip over. Now spread the mint chutney on the other side of both pieces.
Spread the spiced potato mixture on one slice (chutney side up). Add half the amount of cheese and a sprinkle of chaat masala all over. Now top with onion slices, tomato slices, the remaining cheese and another sprinkle of chaat masala.
Place the second piece of bread on top (butter side up). Heat a frying pan greased with a little oil/butter. Once hot, place the sandwich onto the pan and fry on a low heat for 4 minutes. You can add a weight on top (like another frying pan) to help cook and crisp. Flip and repeat on the other side for a further 4 minutes until golden and crispy all over. Enjoy!