Cook potatoes for 6-8 min. in boiling water to cover, then peel, and dice them into half in chunks. Melt butter in heavy 4 litre sauce pan. Add potatoes. celery, parsnips, onions, and carrots. Cook, uncovered, stirring occasionly until lightly browned. Sprinkle with flour, and stir until well coated. Add stock, marjoram, salt, pepper, and mushrooms. Bring to a boil, stirring constantly. Reduce heat to very low, and partially cover pot. Simmer for 25 min. until potatoes are tender. Taste for seasoning...