Add coriander seeds, fennel seeds, cumin seeds, yellow mustard seeds, nigella seeds, fenugreek seeds, black peppercorns and dry red chillies to a pan.
Dry roast the spices on low heat for about 1 minute, or until they become aromatic. Stir continuously so the spices do not burn.
Turn off the heat and let the roasted spices cool slightly. Transfer them to a grinder and grind into a coarse powder. Keep this achari spice mix aside.
Wash the potatoes well and cut them in half. Boil them in water until they are just tender. Do not overcook them, or they may break while frying.
Drain the boiled potatoes and let them rest for a few minutes so the extra moisture dries up.
Heat 1 tablespoon oil in a pan over medium heat. Place the boiled potatoes in the pan, cut side down.
Fry the potatoes for 5 to 7 minutes, or until they turn golden brown and slightly crispy. Remove them from the pan and keep them aside.
In the same pan, add the remaining 1 tablespoon oil.
Add fennel seeds and let them sizzle for a few seconds. Add some asafoetida.
Add chopped onions and chopped green chillies. Cook for 3 to 4 minutes, or until the onions turn soft and light golden.
Add red chilli powder or smoked paprika powder and salt. Mix well and cook for 20 to 30 seconds on low heat.
Add the fried potatoes back to the pan and mix gently so the potatoes are coated with the onion mixture.
Add the prepared achari spice mix and 3 tablespoons water.
Mix gently and cook for 3 to 4 minutes, or until the potatoes absorb the masala and the water dries up.
Once the potatoes are well coated with the thick achari masala, turn off the heat.
Serve hot with paratha, poori, roti, dal-rice or as a snack.