Arugula, Mozzarella, Tomato on Focaccia

large ripe tomatoes, cut into 1/4-inch-thick slices

red onion, thinly sliced

red-wine vinegar

Freshly ground black pepper

packed trimmed arugula, coarsely chopped

Focaccia bread, halved horizontally (enough for 4 sandwiches)

mozzarella, sliced thin (fresh is best)

mayonnaise (optional)