Savory Onion and Mushroom Tart

Ingredients

• Puff pastry (1 sheet, thawed)
• Olive oil (2 tablespoons)
• Onions (2 large, thinly sliced)
• Mushrooms (8 ounces, sliced (cremini or button mushrooms work well))
• Balsamic vinegar (1 tablespoon)
• Fresh thyme (1 teaspoon, chopped (or 1/2 teaspoon dried thyme))
• Salt (1/2 teaspoon, or to taste)
• Black pepper (1/4 teaspoon, or to taste)
• Gruyere cheese (1 cup, grated)
• Egg (1, beaten (for egg wash))

Instructions

1. Preheat the oven to 400°F (200°C).
2. In a large skillet, heat the olive oil over medium heat. Add the thinly sliced onions and sauté until they turn a deep golden brown and caramelized, about 15 minutes, stirring occasionally to prevent burning.
3. Add the sliced mushrooms to the skillet, along with the balsamic vinegar, fresh thyme, salt, and pepper. Cook for another 5 minutes, stirring frequently, until the mushrooms are tender and the vinegar has reduced slightly.
4. On a lightly floured surface, roll out the puff pastry to fit a 9-inch tart pan. Carefully transfer the pastry to the tart pan, pressing it into the edges and trimming any excess pastry from the edges.
5. Spread the caramelized onion and mushroom mixture evenly over the pastry base.
6. Sprinkle the grated Gruyere cheese generously on top of the filling.
7. Brush the edges of the pastry with the beaten egg to achieve a golden, glossy finish.
8. Bake in the preheated oven for 20-25 minutes, or until the pastry is golden brown and the cheese is melted and bubbly.
9. Remove from the oven and allow the tart to cool slightly before slicing into wedges. Serve warm or at room temperature.