Step 1: Caramelize the Onions

Start by creating the flavor base of the tart—the caramelized onions.

• In a large skillet, heat 1½ tablespoons of ghee over medium heat.

• Add the thinly sliced onions, season with salt and pepper, and sauté.

• Stir frequently to prevent sticking or burning. Allow the onions to slowly cook for about 20 minutes, until they become soft and golden brown.

• Pour in the ¼ cup of sherry, and let it bubble away, scraping up any fond from the bottom of the pan.

• Then, add the ¼ cup of beef stock. Let the mixture reduce until nearly all the liquid has evaporated, creating a sticky, deeply flavorful onion compote.

• Remove from heat and set aside.

Step 2: Sauté the Mushrooms

Using the same skillet for depth of flavor:

• Heat the remaining 1½ tablespoons of ghee over medium-high heat.

• Add the sliced mushrooms. Season with salt and black pepper.

• Let the mushrooms cook undisturbed for the first couple of minutes so they get golden brown. Then stir occasionally for about 5–7 minutes.

• Add in the minced garlic and 1 teaspoon of fresh thyme leaves. Cook for another 30 seconds until fragrant.

• Transfer the mushroom mixture to a paper towel-lined plate to drain any excess moisture and cool.

Step 3: Prepare the Puff Pastry

• Preheat your oven to 400°F (200°C).

• Unfold the thawed puff pastry on a piece of parchment paper.

• Roll it out gently to about 10×13 inches, ensuring it’s roughly ⅛ inch thick.

• Score a ½-inch border around the edges with a knife (don’t cut all the way through) to allow a crust to rise.

• Use a fork to prick the center of the pastry all over—this prevents bubbling in the middle while baking.

Step 4: Assemble the Tart

• Sprinkle ¾ cup of Gruyere cheese evenly over the center of the pastry, keeping it within the scored border.

• Spoon the caramelized onions over the cheese, spreading them evenly.

• Next, layer on the sautéed mushrooms, making sure they’re evenly distributed.

• Top with the remaining ½ cup of Gruyere cheese.

• Brush the outer edges of the pastry with the whisked egg—this gives the crust its beautiful golden shine.

Step 5: Bake to Perfection

• Transfer the parchment and assembled tart to a baking sheet.

• Bake in the preheated oven for 26–30 minutes, or until the pastry is golden brown, puffed at the edges, and the cheese is bubbling and lightly browned.

• Remove from the oven and let the tart rest for 10 minutes.

• Sprinkle the top with the remaining thyme leaves for a fresh, herbal finish.