Blend lemongrass, garlic, chillies, ginger, lime leaves, and lime juice in a food processor until smooth.
Heat vegetable oil in a pan and gently fry shallots, salt, and cumin seeds until softened.
Stir in the spice paste and cook for five minutes to release fragrances.
Add beetroot to the pan and cook for two minutes.
Pour in the stock, bring to a boil, and simmer for 8 minutes.
Mix yoghurt with mint, coriander, and cucumber in a small bowl.
Blend the soup with coconut milk in a food processor until smooth and season with salt and pepper.
Reheat the soup briefly and serve in bowls with a swirl of herby yoghurt and flatbread.