3 large vine-ripened tomatoes, cut into 1/4-inch-thick slices
1 red onion, sliced thin
3 tablespoons red-wine vinegar
Freshly ground black pepper
3 cups packed trimmed arugula, rinsed, spun dry, and chopped coarse
Focaccia bread, halved horizontally (enough for 4 sandwiches)
1/2 pound mozzarella, sliced thin (fresh is best)
1 Tbsp mayonnaise (optional)
1 In a large bowl or baking dish combine tomatoes, onion, and vinegar and season with pepper and salt. Marinate 30 minutes.
2 Layer mozzarella, tomatoes, onions and arugula on the focaccia half. Spread some mayonnaise on the top half of the focaccia if desired. Press top half over the bottom, hold together with a couple of tooth picks.
Makes 4 sandwiches.