Heat the olive oil and butter in a large saucepan.
Add the chopped onion and cook until soft.
Stir in the garlic and cook briefly.
Add the celery, potatoes, parsley, and stock.
Bring to a boil, then reduce the heat and simmer for 20–30 minutes until the vegetables are tender.
Blend the soup until smooth using a hand blender or regular blender.
Return to the pan and stir in the double cream.
Season with salt and black pepper to taste.
If desired, stir in crumbled Stilton until melted.
Serve hot with crusty bread and your favourite garnish.