Preheat the oven to 400° F / 205° C. Line a half-sheet rimmed baking tray with parchment.

In a large saucepan, heat the olive oil over medium heat. Add the sliced mushrooms, minced garlic, rosemary, and kosher salt. Cook until mushrooms soften and release their juices, about 4-5 minutes.

Drain and transfer to a bowl. Set aside to cool while you prepare the pastry.

Unroll puff pastry on a rolling mat. With a sharp chef’s knife, trim to approximately 10 x 13 inches. (If your pastry sheet is a square instead of a rectangle, use a rolling pin to roll it a few inches longer.) Flip onto prepared tray.

With a pastry brush, brush the top of the pastry with a thin, even coat of egg wash. Avoid egg-washing the thin edges.

Scatter the grated cheddar cheese on top of the pastry, allowing a 1-inch border around the edges.

Top with the sautéed mushrooms.

With a pastry brush, apply a thin layer of egg wash around the pastry border.

Fold the edges inward, wrapping the pastry around the cheese and mushrooms.

Crimp edges with a fork, then brush the crimped edges with more of the beaten egg.

Sprinkle with grated Parmesan. Season with kosher salt and ground black pepper to taste.

Bake in preheated oven for 30-35 minutes, or until flaky and golden brown. Sprinkle with fresh rosemary and grated Parmesan.

With a sharp chef’s knife or kitchen shears, slice the pastry into squares. Serve warm.

Mushroom puff pastry is best enjoyed on the day it is made. Wrap leftovers in foil and refrigerate for up to two days. Eat cold, or gently reheat in a 350° F oven for 10 minutes, or until warmed through.