Coarsely grind coriander seeds, cumin seeds and ginger in a mortar and pestle. Keep it aside.
Cut the boiled potatoes into cubes and put them in a shallow dish, add in the green peas.
Sprinkle the sesame seed powder and yellow mustard powder in the potato.
Add chopped green chillies, salt, red chilli powder, and the coarsely ground spices (coriander, cumin, and ginger)from the mortar to the potatoes.
Take a pan and heat about 4 tablespoons of mustard oil till it starts to smoke a little. Cooking or heating the mustard oil will remove the pungent raw taste it has.
Switch off the heat and add ½ teaspoon fenugreek seeds and ½ teaspoon turmeric powder to the oil.
Pour this hot oil tempering over the bowl of potatoes and spices and mix thoroughly tossing the seasoning (tadka) with the potatoes.
Squeeze a lemon and toss it again.
Serve it with paratha and chai.