Rinse chickpeas and place in a large bowl. Place onionslices in a separate bowl and sprinkle some salt over them. Let stand 15 minutes or so. Squeeze the onion pieces with your hands and then rinse off under tap water.
Peel and quarter the potatoes and place in a pot of water. Bring to boil and cook until they are firm but cooked through, being careful not to let them get mushy.
Soak tamarind pulp in 1/4 cup water until soft, then rub pulp with fingers until it is dissolved in the water. Strain and pour the runny part into the chickpeas, discarding any seeds or lumpy parts that are left in the strainer.
Place potatoes, tomatoes and onion in the bowl with the chickpeas. If using the cucumber, chiles or spring onion, add them as well. Finally, add the chaat masala, mix well, and serve.
NOTE: If you are using dried chickpeas which have been soaked and softened, add approximately 1 teaspoon of salt to the mixture. The canned chickpeas already have some salt in them.