Heat the oil (or butter) and cook the leek slices until slightly softened but not browned. Add the lettuce leaves and stir until they become soft. Add the water and bring to a boil, then lower the heat and simmer for 10 minutes, stirring occasionally. Stir in the peas and bring back to a boil.

Toss in the mint, then purée the soup, in batches, in a food processor or a blender (or use one of those handy submersible blenders). Season with salt and black pepper to taste.

If the soup has thickened too much, the book suggests adding some warm soymilk.