Peel and chop the onion. Clean and wash the leek and cut into rings. Celery clean, wash and cut into slices. Clean, peel, wash and cut carrots into sticks.
Peel, wash and chop the potatoes. Cut sausage into thick slices. Heat the oil in a large pot and fry the sausage. Add vegetables, pour 1 1/4 litre of water and bring to the boil.
Stir in the stock and simmer for 12 to 15 minutes. Season to taste with salt and pepper. Serve with pesto and garnish with oregano.