Preheat oven to 350° F (176° C). Line a half-sheet rimmed baking tray with parchment.

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

Pass through a fine mesh strainer to drain. Set aside.

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

Add the sliced leeks. Cook over medium-high heat, stirring frequently, until 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 more.

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

Pass vegetable mixture through a fine mesh strainer to remove excess liquid. Discard liquid.

Scoop vegetable filling into a large mixing bowl. Add cooked potatoes and stir to combine. Set aside to cool while you prepare the pastry.

On a silicone rolling mat or lightly floured countertop, unroll one sheet of puff pastry. With a rolling pin roll pastry until large 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!

Arrange on prepared tray.*

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

Scoop filling into a round mound in the center of the pastry, allowing 1 – 1 1/2 inches of space around the outside edge. (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 the second sheet of puff pastry and drape it over the vegetables.

With your fingertips, 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 and can’t seal it properly, 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 45 minutes, or until flaky and deeply golden brown.

Slice and serve immediately. Wrap leftovers in aluminum foil and store in the refrigerator overnight. Eat at room temperature, or rewarm in a 350° F oven until heated through.