beetroot and coconut soup

Ingredient:

• 2 lemongrass stalks
• 2 garlic cloves
• 3 red chillies, deseeded if you don't want too much heat
• 5cm/2in fresh ginger, peeled
• 4 kaffir lime leaves
• 1 lime, juice only
• 1 tbsp vegetable oil
• 2 banana shallots(or 4 round shallots), finely chopped
• pinch sea salt
• 1 tsp cumin seeds
• 500g/1lb 2oz ready cooked beetroot (not the kind prepared with vinegar), chopped
• 600ml/1 pint vegetable stock
• 400ml/14fl oz can light coconut milk
• salt and freshly ground black pepper
• 2 tbsp plain yoghurt
• 1 tbsp fresh mint, chopped
• 1 tbsp fresh coriander leaves
• 5cm/2in cucumber, deseeded, chopped
• flatbread, to serve