Heat oil in a kadhai, add onion and ginger, fry for two minutes.
Add arbi, cover and cook until arbi turns soft.
Add turmeric powder, dhania powder, red chilly powder, salt and tomatoes. Fry for 3-4 minutes.
Then add coconut paste, and fry for not more than one minute. Add water, and when it starts boiling remove from flame.
In another kadhai, add a teaspoon of oil, add mustard seeds, curry leaves and finely chopped garlic. when the mustard starts spluttering and garlic turns red, remove from flame, and add to the arbi curry.
As a variation, You may add some garam masala powder