Mix together salt, garam masala, cumin, coriander, turmeric, and cayenne in a small bowl and set aside.
Place peeled, diced potatoes in a pot of generously salted cold water. The potatoes should be completely submerged with at least 1" of water above them. Bring the pot to a boil, then reduce to a simmer and cook for 12-15 minutes until the potatoes are fork-tender. When the potatoes are done, drain them and set them aside in a large mixing bowl.
Heat a large, high-sided skillet over medium heat. Let the skillet heat up for a couple minutes, then add olive oil and butter. Once the butter is nearly melted, add the diced onion and cook for about 8-10 minutes. Turn the heat down if the onions start to take on darker color. A little color is fine, but the darker they get the sweeter they become.
When the onions are translucent, stir in the spice mixture and add the minced garlic, ginger, and Serrano pepper to the pan. Cook, stirring frequently for 3-5 minutes until very aromatic.
Add the onion mixture to the bowl with the potatoes. Add the cilantro, peas, and lemon juice and stir until the mixture is evenly combined. Some of the potatoes will break apart, but that's a good thing. Taste the mixture and add more salt or pepper if needed.
Take your pie shells and puff pastry out of the freezer about 15-20 minutes before you plan to use them. Use a fork to poke holes all over the two pie shells (this is called docking), including the sides. Divide the potato mixture evenly between the two pie shells, filling them right up to the top. Flatten the mixture in each pie shell so there's not a big heap in the center.
On a lightly floured surface carefully unfold the first puff pastry sheet. It should still be lightly chilled, but not frozen. Use a rolling pin or the side of a wine bottle to roll the sheet out an additional inch or two.
Whisk the egg together with 1 teaspoon water and a pinch of salt. Use a silicone brush to paint the egg wash around the border of the filled bottom pie shells. (See vegan alternative in notes, below.)
Take one of the sheets of puff pastry and drape it over one of the filled bottom pie shells. Use scissors or a knife to trim off any excess pastry, using the edge of the tin as a guide. Press the tines of a fork around the edge to seal the puff pastry and pie shell together.Use a small serrated knife in an up-and-down motion to cut 3-5 long slits in the top of the pie to let steam escape.
Place the pies on a cookie sheet and bake in a 425°F degree preheated oven for 40-45 minutes. When the puff pastry has puffed up and achieved a dark golden brown color, your pie is done. If the top of the pie seems like it's browning too much, drape a sheet of aluminum foil over it in the oven.
Remove pies from the oven and let cool for 20 minutes before slicing and serving.
If you're missing one of the spices just increase the garam masala by that amount to compensate.
To easily peel ginger, use the edge of a spoon.
If the puff pastry is still frozen when you go to unfold it, it might crack or break. Let it sit 10-15 minutes in a warm-ish spot before you try unfolding it again.
Vegan egg wash alternative: Whisk together 2 tablespoon unsweetened non-dairy milk + 1 tablespoon maple syrup or agave + pinch of salt. Or simply use oil instead.
You can freeze the filling or freeze the fully-baked pies. If freezing a fully baked pie, wrap it in saran wrap first and then place it inside a large ziplock bag to prevent freezer burn. To reheat, tent with aluminum foil and place in the oven at 350F degrees for 20-30 minutes before slicing and serving.
If you want to add meat or poultry to your pie, make sure it's fully cooked before you add it. Shredded rotisserie chicken works well here.