Wash lemons thoroughly and wipe with a clean  towel to remove all moisture. Heat half of  gingelly oil and saute lemons in the hot oil, until the skin become tender. Prick the lemon fruits in several  places all around and place them in a bowl.
Make a paste of  ginger, garlic and green chilly.
In a pan, add 3 tsp  of gingelly oil. Splutter mustard seeds and fenugreek. 
When it sputters  completely, add ground ginger, garlic and green chilly paste and curry leaves  into it and saute well until its raw smell disappear. 
Lower the flame and  add red chilly powder and turmeric powder followed by asafoetida powder and  saute for a few seconds.
   
  Heat vinegar  separately in a pan and let it cool for some time and add it to the prepared  masala. Add required salt and sugar.   Taste, and add extra salt /vinegar if needed.
Add the lemon into  the prepared masala, and mix well. If the gravy seems thick, add some boiled  water. 
 Transfer the  whole lemon pickle into a mud jar locally known as bharani or a ceramic jar.  Clean the jar with hot water, and make sure the jar and lid is completely dry  before you use it for storing.
Heat 2 tbsp of gingelly oil, and  allow it to cool and then pour the oil on top of the pickle. This will prevent  fungus growth, and give an authentic taste.
Start using the lemon  pickle (naranga achar) once it settles, after a few days.