German Lentil Soup - cooking recipe

Ingredients

1/2 cup butter (1 stick)
3/4 cup all-purpose flour
12 cups beef stock
8 ounces dried lentils
5 celery ribs, diced
3 carrots, peeled and diced
1 large onion, diced
1 small tomatoes, diced
1/2 lb knockwurst, diced
3 tablespoons dry red wine
1/4 teaspoon white pepper
2 lbs potatoes, peeled and diced
1/4 cup red wine vinegar

Preparation

Melt butter in small saucepan over low heat,whisk in flour and stir mixture 3 minutes; set roux aside.
Bring stock to boil in large saucepan; add lentils and boil until tender, about 1 1/2 hours.
Add celery,carrots and onion to lentils and boil until tender, about 15 minutes;add tomato and reduce heat to low.
Whisk in roux, add diced knockwurst, red wine and white pepper and cook 30 minutes.
Increase heat to high; add potatoes to soup and boil until tender, about 20 minutes,stirring frequently.
Stir in vinegar and serve immediately.