Peel and finely chop the onion. Heat the oil in a large saucepan over a medium heat. Add the onion and cook gently for 3-4 minutes until starting to soften.
Add the cauliflower, stock and bay leaf. Roughly chop the potato (no need to peel) and add this too. Bring to a simmer, then cook for 20 minutes until the vegetables are soft.
Remove from the heat. Take out the bay leaf and blend the soup until smooth.
Stir in the cheese, paprika and mustard. Taste and season with salt and pepper as needed.