Saute� onions, add garlic and ginger. Fry for about 5 minutes. Add tomatoes, and continue frying. Add cumin, coriander and chili powders, and some salt. Fry for another 5 minutes. Add chick peas, boil for a few mintues. Add garam masala, let mixture simmer. Separately, boil tamcon in water until it dissolves. Add this to main mixture. Remove from stove. Serve garnished with coriander leaves and lemon slices.