Heat the oil in a wide bottomed pan. Add the mustard seeds to it.
Once they splutter, add curry leaves and the onions. Add the turmeric to it as well and saute till the raw smell of turmeric disappears. Don't let the onions brown.
Add the cabbage, sprinkle a little water. Cover and cook on low-medium heat, till cabbage is soft and cooked.
Add salt and stir well.
Microwave the thawed frozen coconut for 30 secs or so, till it becomes warm.
Pulse the coconut and green chillies in a food processor till the chillies are broken into the coconut mixture.
Add this to the cabbage mix in the pan and stir in well.
Remove from heat and serve with rice.
I warm the coconut in the microwave so that the natural oil in the coconut blends back in with the coconut. Otherwise the oil separates when frozen, even though you don't see it and it becomes a sticky mess in the food processor and does not pulse or grind well. You can opt out of this step, if you want and skip this step if you are using fresh coconut.