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You'll need a blender or a chutney grinder. You can find chutney grinders at Indian food stores or in some specialty kitchen stores. Add 1 cup of chopped coriander leaves, half an inch of chopped ginger, and one chopped green chili into a blender or chutney grinder. Also, put in one or two teaspoons of lemon juice, half a teaspoon of cumin powder, and half a teaspoon of salt. Add some water and blend or grind the mixture until it is smooth. Check it for taste and add more salt or lemon juice if needed. Cook it until it is easily pierced with a fork, but not completely soft. Once done, drain off the boiling water and run cool water over the potato to cool it down. Then cut it into round slices. Cut the onion into thin, round slices. The tomato and cucumber may be diced into small chunks. It doesn't matter too much how they are sliced, so long as the pieces are small. You can use wheat or white bread for this sandwich. Lay out your bread slices and slather each with butter. Make sure to butter them evenly. Next, spread the chutney on top of the butter. Doing it in this order helps keep the bread from getting soggy. On one side of the slices of bread, add some of the vegetables that you cut up earlier. Sprinkle one to two pinches of chaat masala and cumin powder over the each veggie as you add it. Cover them with the other bread slices and generously spread butter over the top. Chaat masala is a spice used in many types of Indian cooking. If it's not available at your local grocery store, look for it in Indian or Middle Eastern food stores. Make sure that the buttered side is facing down, and then butter the other side. Let the sandwiches grill for two to three minutes, or until the bread becomes crisp.

Summary:
Prepare the chutney. Add the potato to a pot of water and bring it to a boil. Chop up your cucumber, tomato, and onion. Prepare the bread. Add the chopped vegetables. Place the sandwiches on the grill.