01 - Preheat the oven to 375°F (190°C).
02 - In a mixing bowl, combine flour and salt. Rub in cold butter until the mixture resembles coarse crumbs. Gradually add cold water, one tablespoon at a time, mixing until the dough just holds together.
03 - Form the dough into a disk, wrap tightly in plastic wrap, and refrigerate for 20 minutes.
04 - On a floured surface, roll the dough out to fit a 9-inch tart pan. Press the dough evenly into the pan and trim excess edges. Dock the base with a fork.
05 - Line the crust with parchment paper and fill with pie weights. Bake for 12 minutes. Remove the weights and parchment, then bake for an additional 5 minutes until lightly golden.
06 - Reduce oven temperature to 350°F (175°C).
07 - In a bowl, whisk together eggs, heavy cream, and whole milk. Fold in Gruyère, Parmesan, black pepper, nutmeg, and fresh chives if using.
08 - Pour the filling into the pre-baked tart shell and bake for 20 to 25 minutes until the custard is set and the top is golden brown.
09 - Allow the tart to cool for 10 minutes before slicing and serving.