Place chopped green apples in a large bowl. Set aside. Heat oil in a deep pan over medium-high heat. Splutter mustard seeds. Add ginger and garlic. Saute, stirring occasionally, until fragrant. Add curry leaves. Saute for few seconds.

Reduce the heat to low. Add ground turmeric, ground fenugreek, red chilly powder and asafetida. Saute for few seconds. Add vinegar, salt and enough water. Increase the heat, and bring to a boil. Simmer for 2-3 minutes. Remove from heat. Pour the vinegar mixture into the bowl of green apples. Mix well. Once cooled, transfer it to a sterilized container.