Combine all the ingredients except the butter in a large bowl, set aside. Beat the butter until fluffy, then mix with the other ingredients. Transfer to a sheet of cling film and shape into a neat log about 2.5cm in diameter. Wrap in the cling film and refrigerate for at least 1 hour to firm up. This will make more butter than you need for this recipe, but it can be kept in the freezer for up to 3 months: it’s good on steaks, chicken and fish.
Preheat the grill to high. Heat a frying pan over a moderate heat, add a splash of rapeseed oil and the butter. Add the fish and fry for approximately 3 minutes on each side to achieve a golden colour. The fish is ready when the flesh in the centre ‘gives’ a little when pressed with your thumb.
Just before removing the fish from the pan cut circles of the Café de Paris butter and place 3 along the length of each fish. Flash under the grill to melt the butter: by the time the fish reaches the table the butter should be melting all over the fish.