Melt the butter in a pot over medium heat. Place onions in the pot and cook until tender. Stir in celery and carrots, and cook until tender. Stir in the potatoes and flour. Pour in the chicken stock, and season soup with marjoram. Place turkey in the pot, and bring to a boil. Reduce heat to low, cover, and simmer 30 minutes.
Mix the green bell pepper into the soup, and continue cooking 10 minutes, until pepper is tender.