Prepare the green chutney by blending together 1 cup of fresh coriander leaves, 1/2 cup of fresh mint leaves, 2 green chilies, 1/2 inch piece of ginger, 1 clove of garlic, 1/2 teaspoon of chaat masala, 1/2 teaspoon of cumin powder, 1 tablespoon of lemon juice, and salt to taste. Blend until smooth and set aside.
Take 8 slices of white or brown bread and evenly spread butter on one side of each slice.
Spread a generous layer of green chutney on the buttered side of 4 bread slices.
Place thinly sliced vegetables like cucumber, tomato, onion, and boiled potato on top of the chutney layer.
Sprinkle a pinch of chaat masala and black salt over the vegetables.
Cover the sandwiches with the remaining 4 bread slices, buttered side facing down.
Heat a sandwich griller or tawa and grill the sandwiches until they are golden brown and crispy on both sides.
Cut the sandwiches into halves or quarters and serve hot with tomato ketchup or additional green chutney.