Cut the boiled and peeled potatoes into 1 cm sized cubes. Keep aside.
In a heavy bottomed pan, on low heat, toast the sesame seeds until lightly golden. Once cooled, in the chutney jar of the mixer, or a coffee grinder, grind to a fine powder, reserving a pinch of seeds for garnish. Add up to 1/4 cup water and grind to make a paste. Remove from mixer and keep aside.
Grind the coriander stems, garlic, green chilli, turmeric powder and salt to a fine paste, using 3-4 spoons of water. Keep aside.
In the heavy bottomed pan, heat mustard oil. When hot, add cumin seeds.
When seeds splutter, add the coriander stem paste and saute for 1 minute or so.
To this add the cubed potatoes, and stir for 3-4 minutes until well coated.
At this point, add the sesame paste along with 1/2 cup water. Keep the heat on low and bring to a gentle simmer. Allow to simmer for 4-5 minutes.
Remove from heat. Stir in the lemon juice, garnish with coriander and a few toasted sesame seeds.
Serve with rotis, puris or any other flatbread.