Start this recipe by heating the oil in a deep pan and sauté together the onion for 4 minutes to soften. Add in the garlic and ginger paste, frozen coriander leaves and frozen mixed mushrooms. Season to taste with salt and pepper and cook for 4 minutes. Remove the outer layer of the lemon grass and bash with the back of a knife. Add to the pan along with the stock and the coconut milk and bring to the boil. Lower the heat and simmer for just 5 minutes. When ready, remove of the heat and allow to cool slightly before removing the lemongrass. Serve the soup either hot or cold, decorating the top with extra sautéed mushrooms and coriander.