In a heavy 4-quart Dutch oven, melt butter over medium heat and cook sausage 5 minutes, stirring occasionally. Add onions, carrots and celery and cook 5 minutes. Stir in flour, coating vegetables well. Stir in flour, coating vegetables well. Stir in beer and stock and bring to a boil. Lower heat and simmer until soup thickens slightly, about 5 minutes. Add potatoes and cook 10-15 minutes or until tender.
Stir in milk, cheese, thyme, Worcestershire, paprika and tomato paste. Continue stirring until cheese is melted ans soup is smooth. Season with salt, pepper, and Tobasco, if desired. Garnish with scallions. Serves 6-8.