In a bowl, add 3 cups of water and bring it to a boil. Add 350 grams of gunda without removing its seeds into the boiling water. Boil for only 5 minutes on high flame. Then take out the gunda and place it in a sieve to cool down. Once it's cool, remove its seeds and rub some salt in its inner cavity. Keep it aside.
Next, in a mixer jar, add papdi gathiya, roasted peanuts, ginger, and green chilli. Grind coarsely and transfer to a mixing bowl. Add grated mango, sesame seeds, fennel seeds, salt, coriander powder, red chilli powder, cumin powder, garam masala, jaggery, and oil. Mix well. The masala is ready.
Stuff the prepared masala into the gunda and keep it aside.
In a pan, add oil, mustard seeds, cumin seeds, and hing. Once they start spluttering, add turmeric powder and the stuffed gunda. Saute for 3-4 minutes by shaking the pan, but do not stir the gunda with a spatula. Then spread the remaining masala on the gunda.
Lower the flame and add buttermilk into the gravy. Stir continuously until the gravy starts to boil. Cover the pan and cook the sabzi on medium flame till the gravy thickens and releases oil from its sides.
Gunda nu shak is ready to serve with roti or thepla.