Prep the potatoes: Boil potatoes until fork-tender. Peel and cut into 1-inch chunks. Set aside to cool.
Toast spices: In a dry pan, roast cumin seeds, coriander seeds, and fenugreek seeds until fragrant. Cool, then grind to a fine powder.
Heat the oil: In a heavy-bottomed pan, heat sesame oil on medium. Add mustard seeds and let them splutter. Add asafoetida and green chilies.
Add tamarind pulp: Pour in the tamarind extract and cook for 5–6 minutes until slightly thickened.
Season: Add turmeric powder, ground spice mix, and salt. Stir well.
Add potatoes: Gently fold in the boiled potato chunks so they are coated in the spice-oil mixture.
Simmer: Cook on low for 8–10 minutes, stirring occasionally, until the oil begins to separate from the masala.
Finish: Turn off the heat, stir in lemon juice, and let cool completely before storing in a clean, dry jar.