In a large saucepan, melt butter over medium heat. Add the chopped onion and cook until tender without browning. Stir in diced potato and liquid. Bring to a boil and simmer until potato is tender. Stir in your steamed vegetable of choice and return to a simmer. Puree the soup in batches in a blender and return to saucepan. Leave 1 cup of vegetables not pureed if a chunkier soup is preferred. Season soup with salt and pepper and finish with heavy cream. Serve warm with french bread and a salad.