Heat oil in a non stick pan. Now add the cumin and mustard seeds. When they splatter, add the remaining “potato filling: ingredients and mix. Remove from heat and allow to cool.
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 one slice (chutney side up). Add half the amount of cheese. Now top with onion slices, tomato slices and the remaining cheese.
Sprinkle over the chaat masala and place the second piece of bread on top (butter side up).
Melt a small knob of butter on a non-stick frying pan.
Place sandwich on pan and fry for 4 minutes. Flip and repeat on the other side until golden and crispy all over.