Preheat the oven to 400°F.
Slip the grapes out of their skins and put the skins in a separate bowl.
Cook the grape pulp in a saucepan over medium heat, stirring often, until they break down and release their seeds—about 10 minutes.
Push the cooked pulp through a sieve to get rid of the seeds. Mix the seedless pulp back in with the reserved skins.
Add the sugar, cornstarch, lemon juice, butter, and salt. Stir to combine and let the mixture cool for 10-15 minutes.
Roll out your pie crust and line a pie pan, then pour in the grape mixture. Top with the second crust, seal the edges, and cut a few slits in the top.
Brush the top crust with egg and sprinkle on a little sugar. Bake for 35-40 minutes, until the pie is golden and bubbly.
Let the pie cool all the way before slicing. That way, the filling thickens up and slices come out neat.