Preheat the grill to the highest setting.
For the soup, heat the butter in a saucepan and fry the onion until soft but not coloured.
Add the broccoli, potato and stock to the saucepan and bring to the boil. Turn down the heat to a gentle simmer.
Gently simmer the soup for eight minutes, or until the vegetables are soft. Remove from the heat and allow to cool slightly.
Pour the soup into a blender, add the Stilton and pulse briefly. Season with salt and freshly ground black pepper.
For the rarebit, toast one side only of the bread under the grill.
In a small bowl, mix the Stilton, egg yolk and Worcestershire sauce together and spread on the un-toasted side of the bread.
Place the bread on a non-stick baking tray under the hot grill for two minutes, or until the cheese mixture is melted and golden-brown.
To serve, place the rarebit on top of the soup.