Cut off the dark skin of yam with the help of a knife then chop it into 1-1.5 inch chunks. Discard its black portion and place the yam chunks in a bowl filled with water. Grease both the hands with mustard oil while cutting the yam. Wash the yam pieces and place them on a plate.Place a pressure cooker on flame and put the yam chunks into it along with some water. Put the lid on and let the yam cook until the cooker releases a whistle. After that, reduce the flame to low and cook the yam for 2-3 minutes more.After cooking the yam on low flame for 2 minutes, turn off the flame and let the pressure escapes on its own. Once the pressure is released, remove the lid. Yam chunks are cooked aptly, transfer them on a sieve placed over a bowl so that the water gets drained out into the bowl. Allow them to cool down slightly.When the yam chunks cools down slightly, transfer them in a big bowl and mash them finely using a masher.Now add mustard oil, red chilly powder, salt, turmeric powder, crushed carom seeds, ginger paste, asafoetida and squeeze the lemon into it. Mix everything really well. Yam chutney is now ready.Serve this tangy and lip-smacking yam chutney as a side assortment along with any meal. Yam chutney will taste more delicious after 3 days so cover and keep the chutney under sunlight or inside the room for 2-3 days and use it. Store this yam chutney in an air-tight sterilize container. It keep good for 8-10 days when placed outside the refrigerator and 2-3 months when placed inside the refrigerator. Enjoy!