Sweet potato soup

Ingredients

• ½ tsp dried red chilli flakes
• 1 tbsp olive oil
• salt and freshly ground black pepper
• 1.2 litres/2 pints vegetable stock
• 2 large carrots, peeled and roughly chopped
• 4cm/1½ inches fresh root ginger, finely chopped
• 1 garlic clove, crushed
• 700g/1lb 10oz sweet potatoes, peeled and cubed
• 1 onion, roughly chopped