Heat the butter in a pan and add the onion. Sweat without colouring until softened. Stir in the rocket and lettuce, then add the stock and simmer for five minutes. Liquidise in a blender until smooth.
Return to the heat and stir in the crème fraîche and seasoning. Add a little lemon juice if desired. Serve garnished with cubed mozzarella and strips of sun-dried tomatoes.
Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.