Season the flour with salt and pepper and coat the fish, dusting off any excess flour.
Place a large non stick pan over a medium heat. Add the oil and cook the fish for 3-4 minutes on each side until golden. Pop onto a warm plate.
In the same pan add the butter and cook until nut brown, being very careful not to let it burn. Strain through a muslin cloth into a bowl and add the lemon juice and parsley.
Spoon over the fish and serve with the watercress.