Black bean soup

Ingredients

• 1 tablespoon vegetable oil
• 1 onion, finely chopped
• 1 sweet potato, peeled and finely chopped
• 1 clove garlic, finely diced
• 1 teaspoon ground cumin
• ½–1 red chilli, deseeded and finely chopped, plus extra to garnish
• 2 x 380g cans black beans, drained
• 500ml reduced-salt vegetable stock
• 2 tablespoons low-fat sour cream
• Freshly ground black pepper