Heat mustard oil in a pan over medium heat. Add chopped onions and sauté until golden brown.
Add ginger-garlic paste and slit green chilies. Sauté for a minute until fragrant.
Add turmeric powder, red chili powder, coriander powder, and cumin powder. Stir well to combine.
Add tomato puree and cook until the oil separates from the masala.
Add water to adjust the consistency of the curry. Bring it to a gentle simmer.
Carefully add the Rohu fish pieces to the curry. Cover and cook for 8-10 minutes or until the fish is cooked through.
Sprinkle garam masala and garnish with fresh coriander leaves.
Serve hot with steamed rice or roti.