In a small skillet, saute onion in butter for 8-10 minutes or until golden brown. Meanwhile, place potatoes and carrots in a large saucepan and cover with water. Bring to a boil. Reduce heat; cover and cook for 10-15 minutes or until tender. Drain. In a small mixing bowl, mash potatoes and carrots. Beat in onion, sour cream and salt. Sprinkle with chives.