Soak the dried garbanzo beans overnight in cold water with a little baking soda before cooking, or just cook them for a longer time, whichever is more convenient for you.
In either case, simmer them in a large pot with enough water to cover them by at least an inch; if you have hard water, add a pinch of baking soda to the water. When the garbanzos are starting to be tender, add about a teaspoon of salt to the water. The cooking time will vary with the age of the beans—it might take an hour or two, or maybe even longer. They’re ready when they’re tender.
Peel the onions, halve them lengthwise, and cut them in thick slices. Peel and coarsely chop the garlic. Heat the peanut oil and butter in a large, non-stick sauté pan and sauté the onion in it, stirring frequently, until it begins to color. Add the chopped garlic and a dash of salt and keep stirring over medium heat until both onions and garlic are golden brown.
Meanwhile, peel the potatoes and cut them in 2-inch chunks. Toast the cumin seeds lightly in a little pan, stirring and watching until they release their fragrance, then grind them briefly in a stone mortar or a spice grinder.
When the onions and garlic are ready, add the cumin, coriander, cayenne, turmeric, and mustard seeds. Stir over medium heat for about 3 minutes. Add the potatoes, the cooked garbanzo beans, and enough of the cooking liquid from the garbanzos just barely to cover everything in the pan. Bring the liquid to a simmer.
Add ½ teaspoon of salt, the chopped hot green chiles, and the chopped ginger, stir everything, cover the pan, and leave it to simmer for about 15 minutes.
Scald and peel the tomatoes, trim out their cores, and cut them into wedges or big chunks.
Uncover the pan, add the tomatoes, cilantro leaves, fresh lemon juice, and chutney. Simmer the mixture, uncovered, for about another 10-15 minutes. The broth will be thickened, and all the vegetables will be tender, yet still maintaining their separate identities. Taste, and correct the seasoning with a bit more salt or lemon juice if needed.
Serve the stew hot, with a rice pilaf, a chutney or two, and a cool yogurt dish.