Remove all the tough stems and leaves from the broccoli. Cut off the stalks, peel them and cut them into 2.5 cm (1 inch) pieces. Break the florets into very small pieces and set them aside.
Melt the butter in a large saucepan. Add the onion and broccoli stalks and cook, covered, for 5 minutes over a medium heat, stirring frequently.
Add the broccoli florets, potato and vegetable stock to the pan. Bring the mixture to the boil and cook, partially covered, for 5 minutes. Using a slotted spoon, remove 6 or more florets for a garnish and set aside. Season the mixture with salt and pepper and continue to cook for 20 minutes, or until all the vegetables are soft.
Using a blender or food processor, purée the mixture in batches until smooth, transferring each successive batch to a clean saucepan. Chill until required.
When ready to serve, add the cream, lemon juice, Worcestershire sauce and a few drops of Tabasco to the pan. Heat the soup gently and simmer for 3–5 minutes, but do not allow the soup to boil. Just before serving, stir in the grated cheese. Serve the soup, garnished with the reserved broccoli or watercress, if liked.