1Heat 1 inch water to boiling in 4-quart Dutch oven. Add potatoes, green beans and 1/2 teaspoon salt. Heat to boiling; reduce heat to medium. Cover and cook about 10 minutes or until vegetables are tender; drain and set aside.
2While vegetables are cooking, melt butter in 12-inch skillet over medium-high heat. Cook onions in butter 10 to 12 minutes, stirring occasionally, until golden brown. Stir in vinegar, brown sugar and 1/4 teaspoon salt. Pour onion mixture over potatoes and green beans; stir until coated.
3Arrange lettuce on large platter. Arrange warm vegetables on lettuce. Sprinkle with pepper.