Sliced Tomato and Onion Salad

• 1/4 cup balsamic vinegar
• 3 tablespoons water
• 1 teaspoon olive oil
• 1 teaspoon dijon mustard
• 1/4 teaspoon freshly ground black pepper
• 1/8 teaspoon salt
• 5 medium, ripe tomatoes, unpeeled
• 1 large purple onion
• 2 teaspoons minced fresh oregano or 1/2 teaspoon dried