In a small pan heat sesame oil. Add asafoetida and fry. Powder the fried asafoetida and keep aside.
Cut mangoes into small cube sized pieces and put in a mixing bowl. Add 5 tbsp salt and the above powdered asafoetida to the mango pieces. Mix very well. You can transfer the contents into a glass bottle if you wish. Keep covered and refrigerate the bowl/bottle till the next day.
The following day take chilly powder, turmeric powder, fenugreek powder and 1 tsp mustard seeds in a blender. Without adding any water, powder all of this. Spin 3-4 times only.
Empty contents of the blender into a mixing bowl. Add 3 tbsp vinegar to the chilly mixture. Add 1/2 teacup(small cup) water and mix well. Keep this aside.
If the mango is a bit on the sour side then add 1 tsp sugar.
Add the previous days mango pieces and required salt to the wok and mix well. Once mixed well and the mango pieces have warmed, switch off flame immediately. Do not heat further as mango pieces might get cooked.
Take off the stove. Mix well and cool.
Bottle and keep refrigerated. This pickle tastes the best when used after 3-4 days.