In a large dutch oven over medium heat, melt the 3 tablespoons unsalted butter, then add the 3 leeks - white and light green parts only and 3 garlic cloves . Sautee for 5 minutes until the leeks begin to soften. Add the 2 large Yukon gold potatoes and 1 lb asparagus and continue cooking for an additional 10 minutes.
Stir in the 3 tablespoons all-purpose flour until blended. Cook for 1 minute. Add the 1 cup chicken stock and 1 /2 cup milk and bring to a boil over medium heat, stirring constantly; cook for 5 minutes until it starts to thicken.
Remove from heat; stir in 2 teaspoons dijon mustard, 1 ¾ cups ham, ½ cup frozen peas, 1 tablespoon dried parsley, and 1 tablespoon dried thyme. Add salt and pepper to taste.
On a lightly floured surface, unfold 1 sheet puff pastry; roll to ¼-in. thickness. Lay the pastry across the top of the dutch oven. Trim the pastry to the edge of the dish. Seal the edges well so that the filling doesn't bubble over on to the crust.
Cut a few slits to let the steam escape and brush the top with the beaten 1 large egg.
Bake for 30 mins until the puff pastry is golden brown. Allow the pie to cool for 10 minutes before serving. This allows the filling to cool and set up.
Notes
You want the consistency of the ham mixture to be thick but not too thick before placing the ham and leek pie in the oven. The filling will thicken a little more in the oven as the pie bakes.If your filling seems soupy, like heavy cream, let it continue to simmer until it is thick enough to coat the back of a wooden spoon. To speed up the thickening process, scoop out about ¼ cup of the sauce and whisk in a tablespoon or two of flour until no more lumps remain, then whisk this mixture back into the Dutch oven.If the filling seems too thick, whisk in some additional broth to thin it out again.Puff pastry must be completely thawed before using it. Unfolding puff pastry, while it is still frozen, could tear the sheet. The pastry should unfold easily but still be cold after thawing.Use a long, heavy rolling pin to roll out the puff pastry dough. This helps create a pastry that’s even all over and you won’t have to press down too hard to roll it out.Storage: Savory ham pot pie is best the day it is baked. Storing and reheating will cause the crust to become a bit soggy. You can microwave leftovers just know that the pie crust will soften.