In a Dutch oven, saute the potato, leek and onion in oil for 4 minutes. Add mushrooms and cook for 2 minutes. Stir in broth and half of the marjoram. Cover and simmer for 10 minutes or until potato is tender. Cool slightly.
Puree in small batches in a blender; return all to pan. Whisk in sour cream and butter; season with salt and pepper. Heat through but do not boil. Just before serving, sprinkle with remaining marjoram.