In a large saucepan, saute onion in butter until tender. Add the broth, potatoes, salt and pepper; bring to a boil. Reduce heat; cover and simmer for 15-25 minutes or until potatoes are tender. Remove from the heat; cool slightly., Place avocados in a blender; add potato mixture. Cover and puree. Return to the pan; heat through. Garnish with sour cream and bacon.