Heat a pan and add oil to it. The best oil for achar is gingelly oil because it prolongs shelf life. If you don't have gingelly oil use coconut oil.
When the oil is hot over medium heat, add mustard seeds. When mustard starts spluttering add dry red chilies and fenugreek seeds. Fry for 10 seconds and then add garlic, 6-8 green chilies, and curry leaves. You can adjust the quantity of garlic and green chiles based on your preference.
Saute for about 2-3 minutes over medium heat, until the garlic starts to turn light golden color (should not be brown)
Reduce heat to LOW. Add 1/4 tsp turmeric, 1 tbsp chili powder, and 2 tbsp Kashmiri chili powder. Mix all together and roast until the raw flavor of spices settles (about 2 minutes). Add 3/4 cup saved boiled water (water we used to steam gooseberries). Increase the heat to MEDIUM.
Add 6 Tablespoon vinegar and salt, and let it boil. Taste the Gravy and check whether you need to add more salt or vinegar. Add 1/2 tsp roasted fenugreek powder and 1/4 tsp asafoetida powder to this.
Let the Gravy boil again for about 2-3 minutes or until the gravy is thick. Then add gooseberry pieces. Gooseberry is already cooked so there is no need to boil it again. Mix all together and turn off the heat and let it cools down.
Transfer the cooled gooseberry pickles to dry glass bottles.
This can be served in 2-3 days, but if you keep it longer the flavors will absorb into Gooseberry pieces and taste delicious. You can store it in the refrigerator up to 3 months.