French Onion Tart: A Cheesy, Thyme-Scented Pastry Delicacy

Indulge in the rich flavors of a classic French onion tart, a culinary masterpiece that combines caramelized onions, melted cheese, and a flaky, thyme-infused pastry.

Ingredients:

For the Tart:
* 2 large yellow onions, sliced
* 3 tablespoons unsalted butter
* 1 tablespoon olive oil
* 1/4 cup dry white wine (optional)
* 1 teaspoon dried thyme
* Salt and pepper to taste

For the Pastry:
* 1 cup all-purpose flour
* 1/2 teaspoon salt
* 1/2 cup cold unsalted butter, cut into cubes
* 2-3 tablespoons ice water
* 1 egg, beaten

For the Cheese:
* 1 cup grated Gruyère cheese
* 1/2 cup grated Parmesan cheese

Instructions:

Make the Tart:
1. In a large skillet, melt the butter with the olive oil over medium heat.
2. Add the onions and cook for 20-25 minutes, stirring occasionally, until they are deeply caramelized.
3. If using white wine, deglaze the pan by adding it and letting it simmer away.
4. Stir in the thyme, salt, and pepper. Remove from heat and set aside.

Make the Pastry:
1. In a large bowl, whisk together the flour and salt.
2. Add the cubed butter and work it into the flour using your fingers or a pastry cutter until it resembles coarse crumbs.
3. Add the ice water 1 tablespoon at a time, mixing until the dough just comes together.
4. Form the dough into a ball, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

Assemble the Tart:
1. Preheat oven to 375°F (190°C).
2. On a lightly floured surface, roll out the dough to a 12-inch circle.
3. Transfer the pastry to a 9-inch tart pan and trim the edges.
4. Spread the caramelized onions evenly over the pastry.
5. In a small bowl, combine the Gruyère and Parmesan cheeses. Sprinkle the cheese mixture over the onions.
6. Brush the edges of the pastry with the beaten egg.
7. Bake for 30-35 minutes, or until the pastry is golden brown and the cheese is melted and bubbly.

Serving Suggestions:
* Serve warm as an appetizer or main dish.
* Pair with a glass of chilled white wine.
* Garnish with fresh thyme sprigs for extra flavor.

Tips:
* For a richer flavor, use vegetable or chicken stock instead of white wine.
* Add a pinch of sugar to the onions while caramelizing to enhance their sweetness.
* If the pastry starts to brown too quickly during baking, cover the edges with aluminum foil.

Indulge in the irresistible flavors of this classic French onion tart, a culinary delight that will impress your taste buds and leave you craving for more.