Golden Onion Tart with Gruyère and Thyme

Name: Golden Onion Tart with Gruyère and Thyme
Ingredients: For the dough (adapted from a recipe by Alice Waters); 1 cup all-purpose flour; 1/2 teaspoon salt; 6 tablespoons unsalted butter, chilled, cut in 1/4 inch cubes; 3 tablespoons ice water; For the filling:; 2 tablespoons olive oil; 2 pounds yellow onion, peeled and thinly sliced; 1 teaspoon salt; 2 tablespoons port wine; 1 teaspoon freshly ground black pepper; 2 ounces finely grated Gruyère cheese; 1 teaspoon fresh thyme, plus extra for garnish; 1 egg, slightly beaten