Arugula, Mozzarella, Tomato on Focaccia

Ingredients (8)

• 3 large ripe tomatoes, cut into 1/4-inch-thick slices
• 1 red onion, thinly sliced
• 3 tablespoons red-wine vinegar
• Freshly ground black pepper
• 3 cups packed trimmed arugula, coarsely chopped
• Focaccia bread, halved horizontally (enough for 4 sandwiches)
• 1/2 pound mozzarella, sliced thin (fresh is best)
• 1 Tbsp mayonnaise (optional)