- 3 large vine-ripened tomatoes (about 1 1/2 pounds total), cut into 1/4-inch-thick slices
- 1 red onion, sliced thin
- 3 tablespoons red-wine vinegar
- freshly ground black pepper
- 4 cups packed trimmed arugula, washed well, spun dry, and chopped coarse
- Quick Focaccia , halved horizontally with a serrated knife
- 1/2 pound fresh mozzarella, sliced thin
- In a large bowl or baking dish combine tomatoes, onion, and vinegar and season with pepper and salt. Marinate mixture, covered, tossing occasionally with rubber spatula, 30 minutes.
- Transfer tomatoes and onion to a plate with slotted spoon and add arugula to marinade remaining in bowl or baking dish. Season arugula with pepper and salt and toss. Arrange arugula on bottom focaccia half and top with mozzarella, tomatoes, onion, and remaining focaccia half, pressing together gently. Cut sandwiches into 4 pieces.