How to Make It
Preheat the oven to 500°. In a large, heavy skillet, cook the bacon over moderate heat until lightly browned around the edges, about 3 minutes; drain. Add the onion to the skillet and cook over moderate heat until lightly browned, about 2 minutes. Add the potato and cook, stirring, for 3 minutes; add a little water to the pan if the potato begins to stick. Stir in the vinegar, chives and sage and season with salt and pepper; remove from the heat.
On a lightly floured surface, roll out the dough to a 10-by-13-inch rectangle. Transfer to a lightly floured baking sheet, stretching it back into the rectangle.
Stir the créme fraîche and nutmeg into the potato mixture and spread it over the dough, leaving a 1/2-inch border all around. Scatter the bacon on top. Bake the tart for 12 minutes, or until the edges are browned. Cut into squares and serve hot.