Heat the oil in a kadai or pan, add the mustard seeds, asafoetida, dried red chilli and curry leaves and let them crackle. Take care not to burn the tempering.
Keeping the heat on low-medium, toss in the chopped guava, green chillies and garlic. Add the salt, mix well, cover and cook for 10 min till the guava are soft but not mushy.
Now add the ground spices and cook for a further 3-4 minutes. Mix in the jaggery powder and vinegar and cook for a further 2-3 minutes till all the spices are well incorporated.
Check one last time for salt, season if required, turn off the heat and cool the pickle. Store in a glass jar in the fridge for up to 10 days.