Heat oil in a large nonstick skillet over medium-high heat. Add onion to pan; sauté 5 minutes. Reduce heat to medium-low; cook 45 minutes or until golden brown, stirring frequently.
Place potato in a medium saucepan; cover with water. Bring to a boil. Reduce heat, and simmer 12 minutes or until very tender. Drain. Cool 5 minutes. Combine potato and beans in a large bowl; mash with a potato masher. Add onion, vinegar, salt, and pepper; stir well. Add yogurt and sour cream; stir until well blended. Chill at least 1 hour before serving.