Step 1

Preheat oven to 400 degrees F. Line a baking sheet with parchment paper.

Step 2

Place one piece of puff pastry onto the baking sheet and spread with mustard, leaving a 1/2-inch border.

Step 3

Sprinkle half of the shredded cheese over the mustard. Layer ham slices over the cheese, followed by the caramelized onions, thyme, and remaining cheese.

Step 4

In a small bowl, whisk together egg, milk, garlic powder, and parsley.

Step 5

Lightly brush the border of the puff pastry with the egg wash. Place the second square of puff pastry over the top and press the edges to seal.

Step 6

Brush a 1/2-inch border on the top sheet of puff pastry with egg wash. Fold bottom edge of puff pastry over to create a crust of double thickness. Crimp edges with a fork.

Step 7

Brush top of puff pastry with egg mixture (you won't use the entire amount). Cut three, 1- to 2-inch slits in the center of the pie to allow steam to escape while baking.

Step 8

Bake for 25 minutes, until puffed and golden. Let stand for 10 minutes before slicing into squares or strips and serving.