French Onion Soup Quiche with Caramelized Onion Crust

**Prepare the Onion Crust:** In a food processor, combine flour and salt. Pulse a few times to combine. Add cold butter and pulse until the mixture resembles coarse crumbs. Add 1/4 cup of caramelized onions (reserve the rest for the filling). Pulse a few times to combine. Gradually add ice water, one tablespoon at a time, pulsing after each addition, until the dough just comes together. Form the dough into a disc, wrap in plastic wrap, and refrigerate for at least 30 minutes.

**Caramelize the Onions (For Filling):** While the dough chills, caramelize the remaining onions. Heat olive oil and butter in a large skillet over medium-low heat. Add sliced onions and cook, stirring occasionally, until golden brown and very soft, about 30-40 minutes. Be patient and avoid browning too quickly. Stir in brown sugar and sherry (if using) during the last 5 minutes of cooking.

**Preheat the Oven and Prepare the Pie Dish:** Preheat oven to 375°F (190°C). Lightly grease a 9-inch pie dish.

**Roll Out the Crust:** On a lightly floured surface, roll out the chilled dough into a circle slightly larger than the pie dish. Carefully transfer the dough to the pie dish and press it into the bottom and sides. Trim any excess dough and crimp the edges.

**Blind Bake the Crust:** Line the crust with parchment paper and fill with pie weights or dried beans. Bake for 15 minutes. Remove the parchment paper and weights, and bake for another 5 minutes, or until the crust is lightly golden.

**Prepare the Custard:** In a medium bowl, whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper.

**Assemble the Quiche:** Spread the caramelized onions evenly over the bottom of the pre-baked crust. Sprinkle with shredded Gruyere cheese. Pour the egg custard over the onions and cheese.

**Bake the Quiche:** Bake for 30-40 minutes, or until the custard is set and the crust is golden brown. A knife inserted near the center should come out clean. If the crust begins to brown too quickly, cover the edges with foil.

**Cool and Serve:** Let the quiche cool for at least 10 minutes before slicing and serving. Serve warm or at room temperature.