Caramelized Onion and Gruyère Cheese Tart

In a large bowl, combine the flour and salt. Add the cold, cubed butter and rub it in with your fingertips until the mixture resembles coarse crumbs. Gradually add the very cold water, mixing quickly until the dough comes together. Do not overmix! Wrap in plastic film and refrigerate for at least 30 minutes.

In a large skillet over medium-low heat, warm the olive oil and butter. Add the thinly sliced red onions, a pinch of salt, and the optional sugar. Cook slowly, stirring frequently, for 30-40 minutes, until the onions are deep golden brown and very soft. Stir in the chopped fresh thyme, then remove from heat and let cool slightly.

Preheat your oven to 190°C (375°F). Remove the pastry from the fridge and roll it out on a lightly floured surface into a 30 cm (12-inch) circle. Carefully transfer the pastry to a 24-26 cm (9.5-10 inch) tart pan, trim the edges, and prick the bottom with a fork. Blind bake for 10-12 minutes, until lightly golden. Remove from the oven.

In a bowl, whisk together the eggs, heavy cream, whole milk, salt, pepper, and nutmeg. Sprinkle half of the shredded Gruyère cheese over the pre-baked crust. Spread the caramelized onions evenly over the cheese. Sprinkle with the remaining Gruyère cheese, then carefully pour the egg and cream mixture over the top.

Return the tart to the oven and bake for another 30-35 minutes, or until the filling is set and the top is golden brown. Remove from the oven and let it rest for 10-15 minutes before slicing and serving. It's delicious served warm or at room temperature.