Crumble the stock cube into a heatproof jug. Stir in 800ml boiling water. Set aside.
Peel and finely chop the onion. Trim the dry ends off the celery and finely chop it. Peel and dice the potatoes.
Warm a large pan over a medium heat for 1 min. Add ½ tbsp olive oil and the chopped veg. Season well with salt and pepper. Cook, stirring occasionally, for 5 mins till the veg are soft.
While the veg is cooking, break the cauliflower into small florets (see our tip for what to do with the leaves). Add the cauliflower to the pan with the bay leaf. Pour in the stock. Cover and bring to the boil.
When the soup is boiling, turn the heat down and simmer for 15 mins till the cauliflower is soft and tender.
While the soup is simmering, pull the skin off the salmon and flake it into small pieces.
Ladle the soup into a blender and add the milk. Blitz till smooth, or use a hand-held blender in the pan. Taste and add more salt and pepper if you think it needs it.
Ladle the soup into warm bowls. Top the soup with the salmon and the dill leaves. Serve straight away.