Trim, cut, and wash leek. Drain well.
Heat oil in pan. Add leek, and saute.
Take the fish steak. Check for bones, pulling out any you find. If there is a section that is dramatically thinner than the rest, you may choose to cut it off now, and then cook it a shorter time than the rest.
Place the fish skin side down in the pan. Let cook 5 minutes. Stir leeks around.
Turn fish over. (Add thinner piece, if you cut one off.) Cook another 5 minutes, or until done - about 10 minutes per inch of thickness, all together.