I'm guessing it was either broiled or sauteed quickly. For broiling, you would brush with oil and
broil on high. For sauteeing, you would get the pan very hot and cook the fish in oil. Either way, you would use high heat and cook only long enough to brown the outside of the fish before the inside has a chance to dry out. Using a thick cut of salmon and keeping it well chilled before cooking will also help keep it from becoming overdone inside in the time it takes to crisp the outside.