Cut the onion in half, peel it and then cut it into a 5mm (¼") dice.
Cut the celery into a 5mm (¼") dice.
Peel the garlic and slice it as finely as you can.
Melt the butter in a medium-low (20cm or 8") saucepan over a medium heat. Add the onion and celery, then sweat gently for 7-10 minutes without colouring.
Throw in the garlic and fresh sage and stir for 60 seconds.
Pour in the vegetable stock and bring it to a boil.
Add the broccoli and reduce to a simmer, put on a lid and cook until soft. This will take around 12-15 minutes.
Pour the soup into a blender and blitz until silky smooth.
Return to a low heat and stir in the Stilton cheese, be careful not to boil because it burns quickly! Cook until the cheese has completely melted.