Heirloom Tomato Tart

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 pound Jack or Gruyère cheese, shredded (about 2 cups)
1/2 pound plum tomatoes cut into 1/2-inch wedges
1/2 pound medium yellow tomatoes (about 2) or 1/2 pound plum tomatoes, cut into 1/2-inch wedges
1/4 cup Niçoise olives, pitted

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 and olives 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.