Boil 2-3 medium-sized potatoes (approximately 300 grams) until they are soft. Peel and mash them in a bowl.
Heat 2 tablespoons of oil in a pan. Add 1 teaspoon of mustard seeds and let them crackle.
Add 1 finely chopped onion and 1-2 chopped green chilies. Sauté until the onions turn golden brown.
Add the mashed potatoes, 1/2 teaspoon turmeric powder, 1/2 teaspoon red chili powder, 1/2 teaspoon garam masala, and salt to taste. Mix well and cook for 2-3 minutes. Turn off the heat and let the mixture cool.
Take 4 slices of bread and spread a thin layer of butter on one side of each slice.
Place a generous amount of the potato mixture on the buttered side of two bread slices. Cover with the remaining two slices, buttered side down.
Heat a sandwich maker or a non-stick pan. Place the sandwiches in the sandwich maker or on the pan and cook until both sides are golden brown and crispy.
Cut the sandwiches into halves or quarters and serve hot with ketchup or chutney.