In a medium saucepan, heat the vegetable oil over medium-high heat.

Add the mustard seeds and cook until they begin to pop, about 1 to 1½ minutes.

Add the chopped onion and minced serrano chile.

Cook, stirring occasionally, until the onion is soft and caramelized at the edges, about 10 minutes.

Stir in the garam masala, ground coriander, ground cumin, ground turmeric, sweet paprika, and minced garlic.

Cook until fragrant, about 1 minute.

Add the boiled and chopped russet potatoes, thawed frozen peas, and fresh lemon juice to the saucepan.

Break the potatoes up slightly and stir until everything is coated in the yellow turmeric color.

Remove the saucepan from the heat and season the filling with salt and pepper.

Let it cool completely.

The filling can be made and stored in the refrigerator for up to 3 days before assembling the pie.

Preheat the air fryer to 310°F (155°C).

On a lightly floured surface, roll out one sheet of puff pastry into a 10-inch square.

Cut out a 10-inch round from the rolled pastry, discarding the scraps.

Mound the cooled potato filling in the center of the dough round and press and mold it with your hands into a 7-inch disk.

Cut a 7-inch round out of the second sheet of puff pastry (no need to roll).

Place it over the filling disk.

Brush the edge of the bottom dough round with water to moisten, then lift it up to meet the top dough round.

Pinch and fold the edges together all around the filling to form and enclose the pie.

Transfer the pie to the air fryer basket and cut a slit in the top of the pie with a paring knife to vent.

Cover the top of the pie loosely with a round of foil and cook at 310°F for 20 minutes.

Remove the foil and increase the temperature to 330°F (165°C).

Cook until the pastry is golden brown and the filling is piping hot, about 20 minutes more.