Watermelon and Tomato Salad

Ingredients

• 1⁄4 cup champagne vinegar or white wine vinegar
• 3 tsp. sugar
• 1 tsp. kosher salt
• 1⁄4 tsp. cayenne
• 1⁄2 small red onion
• 4-lb. piece of seedless watermelon
• 4 ripe beefsteak tomatoes (about 1 1⁄2 lbs.), stemmed and cored
• 1⁄4 cup extra-virgin olive oil
• 1 loosely packed cup roughly torn basil leaves
• 1 loosely packed cup roughly torn mint leaves
• Kosher salt
• freshly ground black pepper