Wash and make sharp inserts into the arbi. Microwave for 10 mins, check if the arbi is soft, else microwave for another 3 - 5 mins. Allow to cool, peel the skin, and cut into cubes.
Heat a kadai with cooking oil. Add the mustard seeds, urad dal, curry leaves, and chopped garlic. Fry for a couple of minutes, then add finely chopped onions. Saute till the onion turns brown.
Next, add finely chopped tomatoes, along with salt, turmeric powder, chili powder, and coriander powder. Cover with lid and cook in sim for 10 mins for the tomatoes to be done well.
Once the tomatoes are soft, add the cooked arbi, saute well and again cook for 5 mins in high. Simmer with lid covered for 2 mins.
Add the tamarind juice, mix everything. Then, add water, combine all the ingredients. Bring to boil. Cook in high for 5 mins. Then, simmer for 10 mins with lid covered till the gravy is thick.