Heat the oven to 220C. Season the turbot with salt and rub with olive oil before roasting on a tray in the oven for 15-20 minutes. Adjust cooking time if turbot weighs less/more.
To make the Hollandaise sauce put the egg yolks, mustard and water into a bowl and whisk to combine. Place the bowl over a pan of simmering water and whisk until the eggs have doubled in volume and thickened. Whilst still whisking, gradually pour in the melted butter and combine to a smooth and thick sauce. Remove from the heat and add a squeeze of lemon, a little salt and a pinch of cayenne pepper to serve.
Remove the turbot onto a warmed plate and spoon the hollandaise by the side. Serve simply with whatever vegetables you prefer, spinach or dark leafy greens and buttered potatoes are good.