Step 1

Arrange two oven racks to the upper and lower middle positions and preheat the oven to 400 degrees. Line 2 rimmed baking sheets with parchment paper or silicone mats.

Step 2

In a large skillet, melt butter until foaming. Add mushrooms, leeks, and a pinch of salt and sauté until tender, about 10 minutes (the first 7 minutes will be the mushrooms releasing their liquid, and then it will evaporate). Stir in the garlic until fragrant, about 30 seconds.

Step 3

Add havarti cheese, thyme, basil, and the heavy cream. Bring the mixture to a boil and stir until the mixture has thickened, about 2 minutes. Season to taste with salt and pepper (I used just a pinch of salt because the cheese was rather salty). Remove from heat and cool slightly (you'll have about 2 cups filling).

Step 4

Unfold both sheets of puff pastry and place one on each prepared baking sheet. Spoon half the cheese mixture onto the middle of each crust (about 1 cup per crust). Fold the edges over to form the rim of the tart.

Step 5

In a small bowl, whisk the egg and water together to create an egg wash. Using a pastry brush, brush the edges of the each of the tarts. Divide the Parmesan between the tarts, sprinkling it over the top.

Step 6

Bake until the pastry is deep golden brown in color, approximately 20 to 25 minutes. Cool 5 minutes. Serve warm or at room temperature.