Make Dough First. Tart Dough (1st 4 Ingredients): Whisk flour and salt together. Blend in butter with pastry blender until mixture resembles coarse meal. Add ice water one tablespoon at a time, tossing with a fork to make dough. Put dough on work surface. Smear with heel of hand 3-4 times. Form ball, flatten to disc, wrap in plastic. Chill 1 hour. (Dough can be made several days ahead. Allow to soften slightly before rolling.)
In a large heavy skillet, cook onions with salt to taste in oil, covered, over moderate heat for 20 minutes. Remove lid, cook until golden and any liquid has evaporated. Cool slightly.
Preheat oven to 375º. Roll dough into 14-inch round, transfer to 12-inch tart pan or quiche pan. Trim overhang to ¾-inch, fold inward. Spread onions on dough, top with cheese, arrange tomatoes in concentric circles, dot with olives, salt and pepper to taste.