Dust the turbot slices with flour and roast them in butter until golden.
Serve with the mustard sauce (see below) and a ratatouille with fried rocket leaves.
For the sauce, reduce the fish stock by half, add crème fraîche and thicken (assemble) with the ice-cold butter. Heat again, but do not make. Add the mustard and stir well with the whipped cream. Finally season with salt, pepper, a little sugar and a dash of juice of a lemon.