Preheat oven to 425°F. Mix together salt, garam masala, cumin, coriander, turmeric, and cayenne in a small bowl. Set aside.
Parboil the potatoes. Place peeled, diced potatoes in a pot of generously salted cold water. Bring to a boil, then reduce to a simmer and cook for 12-15 minutes until fork-tender. Drain well and transfer to a large mixing bowl.
Sauté the aromatics. Heat butter in a large skillet over medium-low heat. Add diced onion and cook for 8-10 minutes until translucent. Add spice mixture, garlic, ginger, and serrano pepper. Cook for 3-5 minutes until aromatic.
Combine the filling. Add the onion mixture to the potatoes. Add cilantro, frozen peas, and lemon juice. Stir until evenly combined. Taste and adjust seasoning.
Assemble the pies. Divide filling evenly between pie shells. Brush exposed crust edges with egg wash. Unfold puff pastry sheets and drape over each pie. Trim excess and press edges with fork to seal.
Add steam vents. Cut 3-5 slits in the top of each pie. Brush entire surface with egg wash.
Bake. Place pies on a baking sheet. Bake at 425°F for 40-45 minutes until puff pastry is deeply golden brown. Tent with foil if browning too quickly.
Rest and serve. Let cool for 15-20 minutes before slicing. Serve with lime yogurt sauce or chutney.