Blanch tomatoes in a large saucepan of boiling water for about 15 seconds. Cool in bowl of ice water and slip off skins. Cut tomatoes crosswise in half and squeeze out seeds; then finely chop with knife or in food processor. Transfer to mixing bowl and whisk briefly. Whisk remaining ingredients in small bowl until blended. Add to tomatoes and whisk vigorously. Whisk again just before serving. Serve with grilled hamburgers, fish, shrimp or chicken.