How to Make It
In a medium saucepan, whisk 2 tablespoons of the sugar with the cornstarch and salt. Whisk in the egg yolk, then whisk in the milk and vanilla bean and seeds. Bring to a simmer over moderately high heat, whisking constantly, and cook until a loose custard forms, about 3 minutes. Strain into a bowl and press a sheet of plastic wrap directly onto the surface of the custard. Let cool, then refrigerate until chilled, at least 2 hours.
Preheat the oven to 350°. Toast the almonds and pistachios on a rimmed baking sheet for about 8 minutes, stirring occasionally, until lightly browned. Transfer to a rack to cool; coarsely chop the nuts. Leave the oven on.
Butter a 9-inch pie plate. Set 1 sheet of phyllo on a work surface and brush the top with some of the melted butter. Arrange the buttered phyllo in the pie plate and sprinkle with 2 tablespoons of the nuts. Repeat the process with the remaining phyllo sheets and melted butter, and more of the nuts; reserve the remaining nuts.
Poke holes through and around the phyllo layers with a fork. Using scissors, trim the overhanging dough. Bake for about 25 minutes, until the phyllo is golden brown and crisp, rotating the pie dish halfway through baking. Place the dish on a rack set over a foil-lined rimmed baking sheet.
Meanwhile, in a small saucepan, stir the remaining 1 cup of sugar with the water and lemon juice. Bring to a boil over high heat and cook until the syrup thickens slightly, about 4 minutes. Slowly drizzle the syrup all over the phyllo crust, including the edge. Let cool completely.
To serve, pour half of the custard into the crust and sprinkle the reserved nuts on top. Serve the pie sliced into wedges with the remaining custard on the side.