Put the onion, celery, garlic, butter and olive oil into a large saucepan set over a low-medium heat. Gently cook the vegetables for 7–8 minutes until tender but not coloured.
Add a pinch of cayenne to the pan, stir well and cook for a further 30 seconds. Add the cauliflower, potato, bay leaf, milk and vegetable stock. Season with salt and freshly ground black pepper, stir well, half cover the pan with a lid and slowly bring to a gentle simmer. Continue to cook for about 30 minutes, or until the vegetables are tender.
Remove the bay leaf and using a bar mixer or blender, whizz the soup until silky smooth. Add the grated cheese, mustard and Worcestershire sauce, if using, and blend again to thoroughly combine. Taste and add more seasoning as required.
For the cheesy croûtons, preheat the oven to 180C/160C Fan/Gas 4.
Cut or tear the bread into rough peices, tip onto a small baking tray, sprinkle with the garlic granules and season with salt and freshly ground black pepper.
Drizzle over the olive oil and mix well to coat. Slide the tray into the oven and cook the croûtons for about 7 minutes, or until golden-brown and crisp, turning the croûtons over halfway through.
Scatter the cheese over the top of the croûtons and return the tray to the oven for 2 minutes to melt the cheese. Ladle the soup into bowls and scatter with the hot cheesy croûtons and snipped chives and serve immediately.