In a large saucepan, heat the coconut oil. Add the onion and cook until soft, then add crushed garlic and cook for another 2 minutes. Add the stock, water, lettuce, lemon juice and bring to the boil. Reduce the heat and simmer for 15 minutes. Remove from heat and stand 5 minutes before processing soup until smooth with a hand blender or food processor. Return soup to saucepan, add coconut milk and stir over heat until soup is hot.
Season to taste with a little sea salt and freshly ground black pepper.