Blend the tomatoes, green chilies, and ginger to make a puree.
Heat the oil in a saucepan. Test the heat by adding one cumin seed to the oil; if seed cracks right away, oil is ready. Add the asafetida and cumin seeds. When the cumin seeds crack, add the tomato puree, coriander powder, turmeric, and red chili powder. Cook three to four minutes on medium heat. The tomato mixture will separate from the oil and reduce to about half in quantity.
Add spinach, salt, and one-half cup of water. Cook, covered, four to five minutes on medium heat.
Add the chickpeas and mash them lightly with a spatula. Add more water as needed to keep the gravy consistency to your liking.
Cook on low heat for seven to eight minutes. Add the garam masala.