Prepare the cabbage. Wash the cabbage and shred it thinly with a sharp knife or a box grater. Thin strips cook quickly and remain tender-crisp, not mushy. You will need 4 cups of shredded cabbage. Drain excess water in a strainer.
Make the tempering. Heat oil in a large pan over medium-high heat. Once the oil is hot, add mustard seeds, and let them splutter. Add chana dal and urad dal, and fry until light golden.
Add the aromatics. Add curry leaves, red chilies, and green chilies. Fry for 4-5 seconds till fragrant.
Cook the cabbage. Add the shredded cabbage and turmeric powder, then mix well. Turn the heat down to low. Cover and cook for 6-8 minutes, stirring once or twice. Covering the pan steams the cabbage in its own moisture, cooking it through without adding oil or water.
Check for doneness. The cabbage should be tender but still have a bit of bite. It should not be limp or watery.
Season and finish. Add salt and mix well. If there’s any extra moisture in the pan, cook uncovered for a minute.
Add the coconut. Remove from heat and add coconut. Adding the coconut off the heat keeps it fresh-tasting rather than toasted, which is what you want in a poriyal. Taste for salt and adjust if needed.
Serve warm. Serve hot with rice and a South Indian curry of your choice.