Salt the blocks of fish well and set in the refrigerator for one hour.
Get your pan hot over a high heat burner, about 2 minutes. While this is happening, pat the fish dry with paper towels. Make sure it's dry. Add the tablespoon of oil and swirl it around. It will heat up very fast.
As soon as you see a wisp of smoke coming from the oil, lay the fish pieces down in the pan, well spaced, skin side down (or the side where the skin used to be). Turn the heat down to medium-high and let them cook, undisturbed, for 5 minutes, or until you see the bottom starting to caramelize. Using a thin metal spatula, ideally a fish spatula, carefully try to lift the fish from the pan. It should come up easily, or stick in only one or two spots. If not, keep cooking it -- the fish will release when it's ready.
When that happens, add the butter, and once it melts, drop the heat all the way to low. Use a spoon to baste the butter over the pieces of fish until they turn opaque. You will need to tilt the pan to do this properly. Serve the fish seared side up.