Place the potatoes into a medium pot and cover them with water. Place the pot over high heat and bring the water to a boil. Lower the heat and allow the potatoes to cook just until fork tender, about 15 minutes. Drain the potatoes into a colander and return them to the pot.
Coat the bottom of a medium skillet with oil and place it over medium heat. When the oil is hot, add the onion. Sweat the onion for about 5 minutes, stirring occasionally, until it becomes soft and translucent.
Add the garlic, ginger, serrano pepper, garam masala, cumin seeds, ground cumin, turmeric, coriander, cayenne pepper, and asafoetida. Continue to cook the mixture until the garlic and spices are very fragrant, stirring constantly, about 1 minute.
Add the cooked potatoes to the skillet. Stir to coat the potatoes in the spices, then cook everything for about 1 minute, stirring constantly. Remove the skillet from heat.
Lightly mash the potatoes with a potato masher, leaving some chunks.
Stir in the peas, peanuts, lemon juice, salt, pepper, and cilantro. Taste-test the filling and adjust any seasonings to your liking.