Creamy potato and onion soup

4 onions, peeled and chopped
1 pound potatoes, peeled and cut into even-sized pieces
3 tablespoons butter or pure vegetable margarine
1 tablespoon olive oil
salt and freshly ground black pepper
grated nutmeg

Put a quarter of the onions into a large saucepan with the potatoes and 4 cups of water. Bring to a boil, then cover and let cook until the potatoes are completely tender. Meanwhile melt the butter or margarine and oil in another saucepan, put in the remaining onions, cover, and cook gently for about 15 minutes, or until they are very soft. Puree the potato mixture in a blender or food processor, then return it to the saucepan. Add the onions to the saucepan, together with their buttery juices. Season to taste with salt, pepper, and nutmeg, reheat gently, and serve.