Place a large pot on the heat and allow to heat up. Add the mussels and the wine and cover with a lid immediately.
When the mussels begin to open, turn off the heat and allow to sit for 2 mins before removing the lid.
Then strain into a sieve and pick the flesh from the shells discarding the shells but keeping the cooking liquid. Strain the cooking liquid again through a clean damp J-cloth to remove any remaining sand, reduce the stock by half and leave aside until serving.
Dice the carrot and celeriac to an even dice and cook in a boiling salted water separately. When cooked, mix the two vegetables together in an ice bath to stop the cooking process.
For the spinach, pre-cook the spinach in boiling water for 20 seconds, then straight into cold water, strain a squeeze out any excess water. Season and leave in a bowl covered with cling film and a knob of butter to be micro-waved when serving.
To cook the fish, heat up a frying pan until really hot. Season and add the halibut fillets cook until one side becomes a nice golden colour and place in the oven at 200°C for 4 minutes.
While the fish is in the oven, heat up the reduced mussel stock, add the carrot and celeriac, boil for 1-2 minutes ensuring the vegetable is heated through. Then add two large spoons of Crème Fraiche, the chopped herbs and the picked mussel meat.
Boil for further 30 seconds, the remove from the heat and adjust the seasoning.
Divide the mussels mixture evenly into bowls and place a little of the spinach in the centre of each one. Remove the fish from the oven, squeeze a little lemon juice on the top and place onto the spinach.