Roast chestnut soup

Ingredients

• 2 teaspoons olive oil
• 1 large shallot, chopped
• 1 stick celery, trimmed and sliced
• 400g unsweetened, cooked whole chestnuts
• 1 small potato, peeled and chopped
• 500ml reduced-salt stock (either vegetable or chicken)
• 1 teaspoon dried thyme
• 250ml semi-skimmed milk
• Freshly ground black pepper, to taste
• 1 tablespoon low-fat natural yoghurt
• Sprig fresh parsley, chopped (for decoration)