Heat vegetable oil in a pan, add mustard seeds and cumin seeds until they crackle.
Add ginger paste and chopped green chili; sauté for 30 seconds.
Add chopped tomatoes, turmeric powder, red chili powder, and salt; cook until tomatoes turn soft.
Pour in water and jaggery, bringing the gravy to a simmer for 5 minutes.
Fold in thick ganthiya sev right before serving so it absorbs gravy without turning soggy.