Place the dried mushrooms in a bowl with the measured boiling water, then cover and set aside to soak.
Heat the butter and olive oil in a large saucepan or casserole. Add the shallots, celery, potato and garlic, and cook gently for about 10 minutes, until softened.
Squeeze the excess moisture from the soaked mushrooms and chop finely, reserving the soaking liquid. Add both soaked and fresh mushrooms to the pan and cook for a further 4–5 minutes, until softened.
Strain the reserved soaking liquid into the pan along with the vegetable stock, and simmer gently for 8–10 minutes. Remove the pan from the heat and use a hand-held blender to blend until smooth. Season to taste with salt and pepper, then stir in the cream. Ladle into 4 deep bowls. Garnish with chopped chives, sprinkle with extra black pepper and serve with crusty bread, if desired.