Pulse serrano, 4 cloves garlic and ginger in a food processor until minced. Scrape down the sides and pulse again. Add onion; pulse until finely chopped, but not watery.
Heat 6 tablespoons oil in a large saucepan over medium-high heat. Add the onion mixture and cook, stirring occasionally, until softened, 3 to 5 minutes. Add 2 teaspoons coriander, 2 teaspoons cumin and ½ teaspoon turmeric and cook, stirring, for 2 minutes.
Pulse 2¼ cups tomatoes with juice in the food processor until finely chopped. Add to the pan along with ¾ teaspoon salt. Reduce heat to maintain a simmer and cook, stirring occasionally, for 4 minutes.
Add 30 ounces chickpeas and 2 teaspoons garam masala, reduce heat to a gentle simmer, cover and cook, stirring occasionally, for 5 minutes more.
Serve topped with ¼ cup cilantro, if desired.
