Pour the olive oil into a large pan and add the butter, when melted add the potatoes, celery, leeks and fennel. Cut a circle of parchment paper or foil directly on top of the veg, this will intensify the heat and speed up the cooking. Make sure the vegetables do not colour!
When the vegetables start to soften stir in the parsley stalks, sea salt, pepper and enough water to cover everything. Bring to the boil, cover and simmer for about 20 minutes until the vegetables are soft through. Stir in the spinach and parsley leaves, cook until wilted then remove from the heat.
Blitz everything to a fine puree in a liquidizer or with a stick blender. If you like a thicker soup you can leave the fiber in, alternatively you can pass it through a sieve for a fine dining finish.
To make the finishing touches heat the olive oil in a large frying pan, add the sliced mushrooms, sprinkle with sea salt, pepper and the chopped parsley. Set aside.
Serve the soup in warmed bowls, dress with a pile of mushrooms in the centre with a little blob of creme fraiche.
To Finish