Score the fish with a sharp knife and marinate in the lemon juice for about 15 minutes.
Drain, reserving the lemon juice, and dry with kitchen towels.
Melt 75g/3oz butter in a large frying pan, making sure the heat is gentle and the butter doesn't burn.
Cook the fish in the butter for approximately 3-4 minutes on each side.
Increase the heat and pour in the lemon juice from the marinade; allow to bubble and evaporate slightly.
Add the remaining butter to thicken the sauce.