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Break up a beef marrow bone, and cover with cold water. Add half a carrot, two stalks of celery, and an onion, all chopped fine. Simmer until the vegetables are very soft, take out the bone, cool, skim, rub through a sieve, and reheat. Add one cupful of cold mashed potato, a tablespoonful of minced parsley, a tablespoonful of butter, a pinch of soda, and one teaspoonful of cornstarch rubbed smooth in a cupful of cold water. Cook and stir until it thickens, and serve immediately with croutons.