SAVOY CABBAGE, POTATO, BACON AND CARAWAY SOUP

• 200g smoked bacon lardons • 1 tbsp olive oil • 2 onions, finely chopped • 2 celery sticks, finely sliced • 2 garlic cloves, finely chopped • 1 tsp caraway seeds • 25g unsalted butter • 500g waxy potatoes (such as charlotte), cut into 2cm cubes • about 500g savoy cabbage (½ large or 1 small), shredded • 1 litre fresh chicken stock • 4-6 tsp soured cream, to serve