Remove the skin of Arbi and wash it 2-3 times with clean water.
Cut the Arbi into medium thickness pieces.
Heat a pan with 3-4 tablespoons of mustard oil on medium to high flame.
Fry the Arbi in the pan until golden brown color appears.
Prepare the gravy masala by grinding 2 tablespoons of mustard, 10-12 cloves of garlic, 2 dried red chilies, 1 small green chili, 1 teaspoon cumin, and half a teaspoon of whole black pepper into fine paste.
Add half a teaspoon of salt to the fried Arbi and fry for another minute.
Add mustard seeds, fenugreek seeds, and two dried red chilies to the pan and let them cook well.
Fry finely chopped tomato in the pan on medium to high flame until slightly soft.
Turn off the gas and add half a teaspoon of turmeric powder and a teaspoon of red chili powder to the pan. Let it cook for another minute.
Add the mustard paste and mix well with the spices in the pan.
Fry all the spices on medium flame for 4-5 minutes or until oil separates from spices.
Add fried Arbi, salt (as per taste), and water to the pan. Mix well.
Cover the pan and cook on low flame for 12-15 minutes or until Arbi is cooked well.