Heat oil in a saucepan and brown crushed onion on low heat for about 10 minutes. Set aside.
Heat oil in a heavy, non-stick saucepan. On low heat, fry garlic for about 30 seconds.
When it changes color slightly, add coriander, cumin, and red chili powder. Stir for a few seconds. Add splash of water to deglaze the pan.
Add crushed tomatoes. Bring to a simmer, and stir till the sauce thickens, roughly 3 minutes.
Add chickpeas, salt and black pepper. Stir and coat with the spicy tomato mixture for 5 minutes.
Meanwhile, add tamarind to 5 cups of warm water. Dissolve tamarind into water with your fingers. Remove seed, and add tamarind-infused water to the saucepan.
Bring the masala to a simmer, cover with lid, and lower heat. Let chickpeas cook for 45 minutes to 1 hour, stirring occasionally to make sure the chickpeas don't stick to the pan. Add water as needed. The longer they cook, the more they will absorb the spices.
After 1 hour, remove cover. Check for salt and spice. Adjust as needed.
Add crushed onions and green chili. Stir for 5-7 minutes.
Add yogurt and garam masala powder. Stir and remove from heat.