Heat vegetable oil in a pan, add mustard seeds and ajwain until they crackle.
Add ginger paste and crushed green chili; sauté for 30 seconds.
Add diced potatoes, turmeric powder, coriander powder, and salt.
Pour in water, cover tightly, and simmer on low flame for 12 minutes until potatoes are soft.
Stir in grated coconut and jaggery, cook uncovered for 3 minutes, and serve hot with phulkas.