In 3-quart saucepan, heat 1 inch water to boiling. Add potatoes and 1/2 teaspoon salt. Heat to boiling; reduce heat to medium. Cover and cook about 12 minutes or until tender; drain and return to saucepan.
Meanwhile, in 2-quart saucepan, heat 1 inch water to boiling. Add asparagus. Heat to boiling; reduce heat to medium. Cover and cook about 5 minutes or until crisp-tender; drain and add to potatoes.
While vegetables are cooking, in 10-inch skillet, melt butter over medium-high heat. Cook onion in butter about 5 minutes, stirring occasionally, until golden brown. Stir in vinegar, brown sugar and 1/4 teaspoon salt. Pour onion mixture over potatoes and asparagus; stir until coated. Sprinkle with pepper.