In a 4-quart saucepan or Dutch oven, melt the butter over medium heat and add the flour. Cook, whisking, until the mixture is combined and the raw flour taste is gone, about 2 minutes.
Meanwhile, whisk the bouillon cube into the hot chicken stock until combined. Add the stock mixture to the roux in the saucepan, then add the cream. Add the ginger and bring to a simmer, whisking. Cook for 5 minutes. Lower the heat, add the peas and onions, chicken and parsley and season with salt and pepper. Heat until just warmed. Transfer the mixture to a 2-quart ovenproof saucepan or heatproof ramekin. Let cool to room temperature.
Preheat the oven to 400 degrees F. Let the puff pastry thaw just until you can work with it.
Cut the pastry into a circle 1 inch wider than the width of the saucepan or ramekin and poke a few holes in it with a fork. Brush the top 1 inch of the outside of the saucepan or ramekin with half of the egg wash. Place the pastry over top and crimp the edges with your fingers so it sticks. Brush the top of the pastry with the remaining egg wash.
Bake the pot pie until the top is golden brown, 25 to 35 minutes.