Fennel and Onion Galette with Gruyere Crust

Name: Fennel and Onion Galette with Gruyere Crust
Ingredients: For the tart crust; 1-1/4 cups all purpose flour; 1/2 teaspoon Kosher salt; 8 tablespoons unsalted butter, cold; 3 ounces Gruyere, grated; 1/4 cup sour cream; 2 teaspoons fresh lemon juice; 1/4 cup ice water; 1 egg yolk, for egg wash; For the filling; 2 tablespoons unsalted butter; 1 large fennel bulb, core and tops removed, thinly sliced; large yellow onion, thinly sliced; 1 teaspoon fresh thyme leaves removed from the stem; 1 teaspoon brandy; 2 teaspoons dry sherry; 2 teaspoons dry white wine; 1 teaspoon Pernod; salt and pepper, to taste