Firstly you must create a fish stock as the base of the sauce.
Head and shell the King Prawns , reserve them for the stock, and then de-vein the prawns.
Cut the turbot into good size portions and reserve any trimmings.
Trim the bulb of fennel and slice into half inch thick slices - reserve the trimmings.
In a little oil lightly fry a hand full of fennel seeds and half a dozen peppercorns, then add finely chopped bulb fennel trimmings, celery, onion (no skin) and the white part of a leek - up the heat slightly add the prawn heads and shells and fry off until coloured then add a glugg of white wine and reduce the stock by half.
Now add the fish trimmings (and any fish bits / heads etc your fishmonger could provide) along with a splash of water, bring to a boil and skim any scum off the top.
Remove from the hob and add crushed parsley stalks, ripped tarragon, a bay leaf and a squeeze of lemon juice, cover and leave to infuse for half an hour.
Drain the stock and discard all the fish bits and veg.
Put the reserved stock into a saucepan and bring to the boil and reduce.
Melt some butter on the hob in an oven proof pan - place in the fennel slices and fry until slightly coloured - turn and place the pan in a pre-heated oven on medium heat.
In a pan heat some oil with a little butter - when hot add the turbut and cook skin side down until three quarters cooked - place on a warmed plate cover with foil and place in a warm oven.
In the pan the turbut was cooked in deglaze with a small dash of white wine add another small knob of butter and some of the reduced stock and reduce to a sauce.
While the sauce is reducing add a couple of peeled prawns and cook on both sides.
Remove the fish and fennel from the oven - the fish should now be cooked through but not dry.
Place the fennel on the warmed plate with the fish, top with the prawns and spoon over the sauce.