2 large onions (about 1 1/2 pounds), sliced thin
2 tablespoons olive oil
Butter pastry dough for a single-crust 12-inch tart
1/2 lb Jack or Gruyère cheese, shredded (about 2 cups)
1/2 - 1 lb tomatoes cut into 1/2-inch wedges
In a large heavy skillet cook onions with salt to taste in oil, covered, over moderate heat, stirring occasionally, until softened, about 20 minutes.
Remove lid and cook onions, stirring occasionally, until golden and any liquid evaporates. Remove skillet from heat to cool onions slightly.
Preheat oven to 375°F.
On a lightly floured surface with a floured rolling pin roll dough into a 14- inch round (about 1/8 inch thick). Fold round in half and transfer to a 12-inch tart pan with a removable fluted rim or a 12-inch quiche dish. Unfold dough, easing to fit, and trim overhang to 3/4 inch. Fold overhang toward center and press against side of pan or dish. Spread onion mixture over dough and top with cheese. Arrange tomato wedges in concentric circles over cheese and season with salt and pepper.
Bake tart in middle of oven 1 hour, or until pastry is golden, and cool on a rack. Remove rim of pan if necessary. Serve tart warm or at room temperature.
Butter Pastry Dough
2 cups all-purpose flour
1 1/2 teaspoons salt
1 1/2 sticks (3/4 cup) cold unsalted butter, cut into bits
6 to 7 tablespoons ice water In a large bowl whisk together flour and salt and with a pastry blender or fingertips blend in butter until mixture resembles coarse meal. Add ice water, 1 tablespoon at a time, tossing with a fork to incorporate, until mixture begins to form a dough. On a work surface smear dough in 3 or 4 forward motions with heel of hand to slightly develop gluten in flour and make dough easier to work with. Form dough into a ball and flatten to form a disk. Wrap dough in plastic wrap and chill 1 hour. Pastry dough may be made 1 week ahead and chilled.