Thai red sweet potato soup

Name: Thai red sweet potato soup
Ingredients: 1 tbsp. olive oil; 150 g (5oz) shallots, roughly chopped; 2 garlic cloves, finely chopped; 2 tbsp. Thai red curry paste (we used Thai Taste); Small bunch coriander, leaves and stalks chopped separately; 700 g (1 ½lb) sweet potatoes, peeled and cut into 3cm (1¼in) chunks; 1 stalk lemongrass, halved lengthways; 900 mL (1 ½ pint) hot vegetable stock; Zest and juice 1 lime, plus extra wedges to serve; 200 mL (7 fl oz (⅓ pint)) coconut milk; 1-2 tsp Thai fish sauce (optional); 2 spring onions, finely sliced; 25 g (1oz) salted roasted peanuts, roughly chopped; Chilli oil and chilli flakes (optional), to serve