Slow-cooker German potato soup (kartoffelsuppe) | Jamie Oliver

Name: Slow-cooker German potato soup (kartoffelsuppe) | Jamie Oliver
Cuisine: European, German
Ingredients: Olive oil; 1 onion, peeled and chopped; 1 stick celery, finely chopped; 2 carrots, peeled and cubed; 1 large leek, washed, and chopped; 1.5kg/3lb potatoes, peeled and chopped; Half a head of sweetheart cabbage, sliced; 1 tablespoon marjoram; Sea Salt and freshly ground pepper; 1 litre/4 cups vegetable stock; A small piece of German sausage, to garnish (optional); Fresh parsley