1. Trim the meat of as much fat and sinew as possible. Mix ginger and garlic paste together; rub onto meat and let it sit at room temperature at least 30 minutes.
2. Put leeks in a large bowl of water, swish them around, and let the dirt settle. After a couple of minutes, lift leeks from water and put in a colander to drain.
3. In a deep, heavy 12-in. pot or a 12-in. frying pan with sides at least 2 in. high, heat oil over medium-high heat. Add chiles. When they start to look blistered, add cumin seeds; stir for a few seconds, then add leeks.
4. Lower heat to medium and cook, stirring from time to time, until leeks soften, about 10 minutes. Push leek mixture to the side and add meat, tossing it constantly so that it changes color without burning or sticking to the pan. Add splashes of water as necessary to keep things from sticking.
5. Pour in about 7 cups water (or enough to almost cover the lamb). Add 1 tsp. salt, turmeric (if using), and 2 cups cilantro. Bring to a boil, reduce heat to maintain a simmer, cover, and let meat cook until it's meltingly tender but not in shreds, 1 1/2 to 2 hours. Taste for salt and skim excess fat from gravy (see Notes).
6. Just before serving, transfer shanks to a plate and bring gravy to a boil. Cook until reduced by half, about 20 minutes. Return shanks to pan, add peas, cover, and cook until warmed through, about 5 minutes. Stir in remaining 1/2 cup cilantro and serve.
Note: Nutritional analysis is per serving.