Pick over, wash, and drain the chickpeas. Soak the chickpeas in 7 1/2 cups of water for 20 hours.
Put the chickpeas and their soaking liquid into a large pot and bring to a boil. As they come to a boil, a white foam will emerge on the surface. Skim off the foam with a ladle.
Cover, lower the heat, and simmer gently for an hour and a half, or until chickpeas are tender. Strain the chickpeas and save the cooking liquid.
Finely chop the green chili. Grate the ginger. In a small bowl or teacup, combine the chili, ginger, lemon juice and 1/2 teaspoon salt. Mix well and set aside.
Finely chop the onions. Finely chop or puree the tomatoes. Heat the oil in a heavy, wide pot over medium-high flame. When hot, add the chopped onions. Stir and fry for 8-10 minutes, or until the onion bits develop reddish-brown spots.
Add the tomatoes. Continue to stir and fry another 5-6 minutes.
Put in the coriander, cumin, and turmeric. Stir and cook for about 30 seconds.
Now add the drained chickpeas, 1 3/4 ( 400 ml) of their cooking liquid, 2 teaspoons of salt, the garam masala, and cayenne.
Stir to mix and bring to a simmer. Cover, turn the heat to low, and cook very gently for 20 minutes. Stir a few times during this period.
Add the lemon mixture to the chickpeas. Stir again to mix. Serve hot or lukewarm.