Prepare the Caramelized Onions

1. Heat 1 ½ tablespoons of ghee or butter in a large skillet over medium heat.
2. Add the onions, season with salt and black pepper, and cook, stirring occasionally, for 20 minutes or until deeply caramelized.
3. Stir in the sherry and beef stock, cook until liquid is mostly evaporated, then remove from heat and set aside.

Sauté the Mushrooms

1. In the same skillet, melt the remaining 1 ½ tablespoons of ghee or butter.
2. Add the mushrooms, garlic, thyme (reserving some for garnish), salt, and pepper. Sauté until golden and fragrant, about 8 minutes. Set aside.

Assemble the Tart

1. Preheat your oven to 400°F (200°C).
2. Roll out the thawed puff pastry on a lightly floured surface and transfer to a parchment-lined baking sheet.
3. Score a 1-inch border around the edges of the pastry without cutting through.
4. Sprinkle half of the Gruyere cheese inside the scored border. Top with the caramelized onions, sautéed mushrooms, and remaining Gruyere.

Bake the Tart

1. Brush the edges of the pastry with the egg wash for a golden crust.
2. Bake for 26–30 minutes or until the pastry is puffed and golden brown.

Garnish and Serve

1. Allow the tart to cool slightly. Garnish with the remaining thyme leaves.
2. Slice into 8 pieces and serve warm or at room temperature.