Cook potatoes in boiling water to cover and salted with 1 teaspoon salt in a covered Dutch oven 20 minutes or until tender; drain and cool.
Combine potatoes, bell pepper, and next 3 ingredients in a large bowl.
Whisk together remaining 1 teaspoon salt, vinegar, sugar, and pepper until smooth. Add oil in a slow, steady stream, whisking constantly. Pour over potato mixture, and toss gently to coat.
Cover and chill 1 hour. Drain; serve over lettuce.