How to Make It
Working over a medium bowl, grate the butter on the large holes of a box grater; freeze. In another bowl, combine the flour, sugar and salt. Using a pastry blender or 2 knives, cut in the butter until the mixture resembles coarse meal. Add the cream and mix with a fork until a dough forms. Turn the dough out onto a lightly floured work surface and pat into a disk. Wrap in plastic and refrigerate until chilled, at least 1 hour.
In a large skillet, cook the bacon over moderate heat, turning, until crisp, about 6 minutes. Drain the bacon on paper towels and cut into 1-inch pieces. Pour off all but 3 tablespoons of the fat from the skillet. Add the onions to the skillet and cook over low heat, stirring occasionally, until caramelized and very tender, about 1 hour. Season with salt and pepper and let cool.
Preheat the oven to 425°. On a lightly floured surface, roll out the dough to a 15-inch round. Fold the dough in half and transfer it to an 11-inch fluted tart pan with a removable bottom. Unfold the dough and press it into the pan. Trim the overhang. Freeze the tart shell until firm, about 10 minutes.
Line the tart shell with foil and fill with pie weights or dried beans. Bake for about 30 minutes, or until the pastry starts to dry. Remove the foil and weights and bake for about 3 minutes longer, or until golden brown. Transfer the tart shell to a rack to cool.
Turn the oven down to 375°. In a bowl, whisk the eggs with the cream. Stir in the bacon, onions, Gruyère, Parmesan, rosemary, cayenne and nutmeg. Season with 1/2 teaspoon salt and 1/4 teaspoon pepper. Pour the filling into the tart shell and bake for 25 minutes, or until the custard is set. Transfer to a rack to cool slightly. Unmold and serve.