Thai chicken and sweet potato soup

Name: Thai chicken and sweet potato soup
Ingredients: 1 tsp olive or rapeseed oil; 2 garlic cloves, chopped; 1 red chilli, deseeded and chopped; 2cm chunk root ginger, chopped; 1 stalk lemongrass, bashed; 1 x 25g pack coriander, leaves and stalks chopped separately; 2 tbsp red Thai curry paste; 750ml chicken stock (made with 2 stock cubes); 1 small can (160ml) coconut cream; 500g sweet potato, peeled and roughly chopped; 2 skinless chicken breasts, sliced; 1 lime, juice only; 1 tsp sugar; ½ tsp fish sauce; crusty bread, to serve (optional)