Presoak peas, if necessary, according to package directions. Drain well, if presoaked. In a large pot, add water or stock, whole onion, carrot, turnip or parsnip, marjoram, thyme, and salt. Cook until peas and vegetables are tender, about 1 1/2 to 2 hours. Drain well. Mash peas and vegetables in blender or press through a sieve. In a small frying pan, saute the minced onion in butter until lightly browned; blend in flour and cook about 2 minutes. Add to blended peas and vegetables. Beat until fluffy and serve hot.