Preheat oven to 180° C / 350° F. Line a half-sheet rimmed baking tray with parchment.

With a knife, dice the potatoes into 1/2-inch cubes.

Bring a pot of salted water to a boil. Add diced potatoes and cook for 5-7 minutes, or until fork tender.

Drain potatoes and set aside.

Meanwhile, slice the leeks (white and pale green parts only) into thin rings.

Slice the mushrooms.

Add the olive oil to a large frying pan. Add the garlic and sautée for 1-2 minutes.

Add the sliced leeks to the pan. Cook over medium-high heat, stirring frequently, until leeks are soft and fragrant, about 5 minutes.

Add the mushrooms and a pinch of salt and sautée until mushrooms release their juices, about 2-3 minutes.

Add the vegetable bouillon, water, thyme, rosemary, nutmeg, and red pepper flakes. Sautée 2-3 minutes more. Season with salt and pepper to taste.

Transfer vegetable mixture to a fine mesh strainer and strain to remove excess liquid.

Transfer to a large mixing bowl. Toss in cooked potatoes and set aside to cool whiile you prepare the pastry.

On a silicone rolling mat or lightly floured countertop, unroll one sheet of puff pastry. Use a rolling pin to roll pastry until it is big enough to cut a disc no smaller than 9 inches in diameter (roughly the size of a dinner plate). Just measure for now—do not cut the pastry!

Transfer pastry to prepared baking sheet.

Add the shredded cheeses to the vegetable mixture and toss to combine.

Scoop vegetable mixture into a round mound in the center of the pastry, allowing 1 – 1 1/2 inches of space space around the edges. (Depending on the size and shape of your puff pastry sheet, you might have a few extra inches of pastry on both sides.*)

With a pastry brush, apply a thin strip of beaten egg around the vegetable mound.

Roll out the second sheet of puff pastry and place it atop the vegetable mixture.

Use your fingertips to seal the pastry around the vegetable mound. When finished, it should look something like this:

With a paring knife, cut the pastry into a circle shape. (I like to use a 9-inch pastry ring or bowl as a guide, but you can freehand it, too!). Be sure to allow a 1-inch border between the vegetable mound and where you cut the pastry—if you cut too close to the filling, you won’t be able to properly seal the pithivier and it will leak while baking.

With a pastry brush, apply a layer of egg wash to the pastry.

Use a knife to scallop the edges, or crimp with a fork.

If desired, tint the remaining egg wash with red gel food coloring.

Once first coat of egg wash is completely dry (it should have a matte appearance) use a pastry brush to apply a second layer of egg wash.

With a skewer or toothpick, poke a small hole in the center of the pastry, creating a place for air to escape.

Use the back of a paring knife to score the pastry as desired.

Bake in preheated oven for about 40 – 45 minutes, or until pastry is flaky and deeply golden brown. Slice and serve immediately.

Pithivier is best enjoyed on the day it is made. Wrap leftovers in aluminum foil and store in the refrigerator overnight. Enjoy leftovers at room temperature, or place on a parchment-lined tray and gently rewarm in the oven for 5-10 minutes at 350°F.  Leftovers can also be frozen.