For the turbot:
Bring the water to a boil then whisk in the salt to dissolve. Add the sea lettuce and chill overnight.
Portion the turbot to 120g pieces (2 pax) and some 60g pieces (1 pax). Place in the brine for 8 mins then remove and drain on J-cloth.
For the mussels and mussel stock:
Debeard then wash the mussels. Drain well and spread onto 2 gastros. Set the Rational to 100C full fan steam.
Bring the wine and water to a boil, pour over the mussels, then place in the oven and cook for 2-3mins until just opened. Cook as little as possible.
Pour through a colander, then pick out the meat from the shells.
Strain the juice through a double layer of J-cloth and keep the mussels in a small amount of the liquor just to keep dry.
For the finished sauce (2 days):
Sweat the shallots in olive oil and salt, lid on, for 15 mins until cooked.
Add the mushrooms, salt and lemon juice, mix, cover with lid and cook for another 10 mins.
Add the Noilly Prat, bring to a boil, then add the mussel stock and double cream and bring to a boil.
Reduce heat, simmer for 5 mins then pass, squeezing on the mushrooms and shallots.
Thicken with the xanthan gum, then transfer 750g of the sauce to a blender, add the nasturtium and garlic, and blend for 30secs and repeat until all the sauce has been used up. Blast chill.
For the yellow courgette (per service):
Slice the courgette into thin rounds across using a mandolin, 1mm thick.
Pickle before service, raw, then strain off.
To assemble:
Steam the turbot at 75C until just cooked (45 C core). Carve down the middle to show the cuisson.
Place the 8 courgette slices, overlapping neatly in a straight line in a bowl, then top with the turbot.
Scatter over 2 nasturtium leaves and 1 yellow flower.
Heat the sauce until piping hot and pour over at the table.