In a large pot, bring water to a boil. Add the octopus pieces and cook for about 20-30 minutes or until tender. Drain and set aside.

Heat vegetable oil in a large skillet over medium heat. Add mustard seeds and let them splutter.
Add chopped onions, garlic, ginger, and green chilies (if using). Sauté until the onions are soft and translucent.

Stir in the chopped tomatoes and cook until they soften and release their juices.
Add turmeric powder, chili powder, ground coriander, and ground cumin. Cook for a couple of minutes until the spices are fragrant.

Pour in the vinegar and let it simmer for a few minutes.
Add the cooked octopus pieces to the skillet and stir to coat them with the spice mixture. Cook for another 5-10 minutes to allow the flavors to meld.

Season with salt to taste and garnish with fresh coriander leaves.
Serve hot with rice or bread.

Enjoy it!