• Blend flour and salt together in a large bowl. Cut in the butter using a pastry blender (or pulse in a food processor) just until most of mixture resembles coarse meal with some small (roughly pea-size) butter lumps. Mix the vinegar and 5 tablespoons of ice water together and drizzle evenly over the flour mixture and gently stir with a fork
• Squeeze a small handful: If it doesn’t hold together, add more ice water, ½ tablespoon at a time, stirring (or pulsing) until just incorporated, then test again. (Do not overwork mixture, or pastry will be tough.)
• Turn out mixture onto a lightly floured surface. Gather dough together with pastry scraper and press into a ball, and then flatten into a disk. Chill dough, wrapped in plastic wrap, until firm, at least 1 hour.
• Roll out dough on a floured surface with a floured rolling pin into a 15-inch round and fit into a 11″ tart pan, lifting and gently pressing into the pan. Trim excess dough, leaving a ½-inch overhang, then fold overhang over pastry and press against side to reinforce edge. Lightly prick bottom with a fork and chill until firm, about 30 minutes. (If using a tart pan with a revomable bottom place the pan on a baking sheet when putting it into the oven).
• Put oven rack in middle position and preheat oven to 400°F.
• Line chilled shell with foil and fill with pie weights. Bake until pastry is set and pale golden along rim, 15 to 20 minutes. Carefully remove foil and weights and bake shell until golden all over, 10 to 15 minutes more. Transfer baked shell to a rack to cool. (Reduce oven temperature to 375 degrees)

Filling:

• Place the bacon into a cold skillet, and cook over medium heat until browned. Remove bacon from skillet and drain on paper towels, reserve 4 tablespoons of the bacon fat.
• Add sliced onions to the skillet with the reserved bacon fat and cook onions with ¾ teaspoon salt and ½ teaspoon pepper over moderate heat, stirring, until just wilted, about 2 minutes. Cover skillet with a lid and continue to cook, stirring frequently until onions are very soft and pale golden, about 20 minutes. Stir in bacon, then remove from heat and cool completely.
• Whisk together the cream, eggs and yolks, nutmeg, thyme, remaining ½ teaspoon salt, and remaining ½ teaspoon pepper in a large bowl, then stir in onions.
• Pour filling into tart shell, spreading onions evenly, sprinkle ½ cup of grated Gruyere cheese evenly over the top and bake until filling is set and top is golden, 40 to 50 minutes. Serve warm or at room temperature