• 8 anchovy fillets, rinsed and patted dry (see Note)
Put flour and 1/2 teaspoon salt in mixing bowl. Drizzle in 1/4 cup olive oil, egg and 1/4 cup cold water, then mix quickly with hands to make a soft dough. If dough seems dry, add more water, 1 tablespoon at a time. Form dough into a rough rectangle, wrap and refrigerate for 30 minutes.
Preheat oven to 425 degrees. On a flour-dusted surface, divide dough in half. Roll out 1 piece to a rectangle approximately 9 by 12 inches (reserve other piece for future use, or to make a second tart).
Stir 2 tablespoons olive oil and the garlic together in a small bowl. Put rolled dough on lightly greased baking sheet. Drizzle surface of dough with garlic oil. Scatter onion slices over dough. Arrange tomato slices over dough in 1 layer without crowding, then season with salt and pepper. Top tomatoes with mozzarella slices in 1 layer. Tuck olives and capers here and there, if using. Drape top of tart in spots with anchovy fillets.
Bake for 6 minutes on bottom shelf of oven, then transfer to top shelf and bake for another 6 minutes or until nicely browned. Remove from oven, let cool slightly, then cut into squares. Serve warm or at room temperature.