Preparation Steps

• For the pastry, whisk together flour and salt in a large bowl. Cut in the cold butter using a pastry blender or your fingertips until the mixture resembles coarse crumbs. Gradually add ice water, 1 tablespoon at a time, mixing until the dough just comes together. Form into a disc, wrap in plastic wrap, and refrigerate for at least 30 minutes.
• For the filling, heat olive oil and butter in a large skillet over medium heat. Add the sliced onions, salt, and pepper. Cook, stirring occasionally, for 40-50 minutes, or until the onions are deeply caramelized and golden brown. Stir in the beef broth and cook for another 5 minutes until slightly reduced.
• Preheat oven to 375°F (190°C). On a lightly floured surface, roll out the chilled dough into a 12-inch circle. Carefully transfer the dough to a 9-inch tart pan with a removable bottom, pressing it into the bottom and up the sides. Trim any excess dough.
• Spread the caramelized onions evenly over the bottom of the tart crust.
• In a medium bowl, whisk together the eggs, Gruyère cheese, Swiss cheese, and thyme. Pour this mixture evenly over the onions.
• Bake for 30-40 minutes, or until the filling is set and the top is golden brown. Let cool slightly before slicing and serving.