6 ounces meaty slab or thick-cut bacon, cut into 1/4-inch pieces
1 medium onion, thinly sliced
1 medium baking potato, peeled and coarsely shredded
2 teaspoons sherry vinegar
2 teaspoons snipped chives
1 teaspoon chopped sage
Salt and freshly ground pepper
1/2 pound prepared pizza or bread dough, thawed if frozen
1/4 cup crème fraèche
1/8 teaspoon freshly grated nutmeg
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.
The tart can be baked earlier in the day; reheat before serving.