For Potato Masala
1. Heat oil in a nonstick pan, add mustard seeds, chopped green chilli, curry leaves let it splutter.
Add boiled potato and mash well.
2. Add asafoetida, turmeric powder and salt to taste and mix everything well and cook for a few minutes.
3. Add coriander leaves and green peas, mix well and cook for more 2 minutes. Once it's done, transfer it into a bowl.

You can use readymade frozen green chutney. Mix in chaat masala.

But if you want to make at home:
Green chutney
1. In a grinder jar, add ginger, green chilli, coriander leaves, mint leaves, ice cube and water as required grind it into a smooth paste.
2. Transfer it into a bowl, add salt to taste, chaat masala and mix well.

Sandwich
1. Spread butter evenly on the bread slices. Spread the coriander chutney.
2. Spread the potato masala filling on the bread slices.
3. Place the slices of onion, tomato, capsicum, etc.
4. Sprinkle a pinch of chaat masala.
5. Place the cheese slice on top.

Grill on pan with lots of butter. Serve with ketchup on the side.