Heat mustard oil in a kadai or a wide, deep cooking pot over medium heat until it turns slightly smoky. Using a wide pan is essential, as shredded cabbage initially takes up a lot of space. It also helps prevent excessive steaming, ensuring the cabbage stays crisp rather than turning mushy.
Lower the heat, add cumin seeds, hing, and ginger; fry for 10-15 seconds to scent the oil with aroma of whole spices.
Add the sliced onions and fry over low-medium heat for 3-5 minutes until they turn soft and golden (but not too dark, as they may become bitter). Do not stir onions continously else they start breaking as they soften.
Next, add the ground spices—turmeric powder, coriander powder, and red chili powder, along with about ¼ cup of water. Fry until the raw aroma of the spices dissipates and the water evaporates, leaving you with a thick masala.
If using fresh green peas, add them to the masala and mix well. Pour in about ¼ cup of water, reduce the heat to low, cover the kadai, and cook for 4-6 minutes until the peas soften.
Uncover and add the cabbage. If using frozen peas, add them along with the cabbage. Stir well to coat the cabbage with the masala. Fry on medium heat, stirring occasionally. At first it will deepen in color but as it softens, it will acquire the color of turmeric.
Make sure that the cabbgae remains slightly crunchy while few shreds acquire a caramalized color.As the cabbage wilts, continue frying until it softens but remains slightly crunchy. I prefer not to cover the kadai, but you can if needed to cook the cabbage down further.
During the last 2-3 minutes, increase the heat to high and bhuno the sabzi to develop a deep, nutty flavor. Sprinkle garam masala and amchur powder, then give it a final mix. Turn off the heat, garnish with chopped cilantro, and serve.