Melt 1/2 cup butter. Add 2 lbs. thinly sliced onions; cook until soft. Add 1 1/2 teaspoons of paprika and 6 cups beef stock. Heat 1/2 cup vegetable oil. Stir in 3/4 cup of flour to make a roux (paste). Add to stock, stirring constantly. Stir in 3/4 teaspoon of celery salt, salt and pepper. Simmer covered for 2 hours. Just before serving, add 1 cup beer. Heat but do not boil (optional). Place croutons or sliced French bread topped with Parmesan cheese and 1/2 cup toasted in bottom of each bowl. Put soup on top. Cover with slice of Mozzarella cheese. Broil until melted. Makes 12 servings.