Preheat oven to 400°F. Heat a large, dry cast iron fry pan over high heat until it literally begins to smoke - about 10-15 minutes. Don't worry, you can't get the pan too hot. Brush filets on both sides with oil. Sprinkle Cajun seasoning generously on both sides of filets. Place into heated pan and pour melted butter directly over filets. Be careful here, as the butter may flare up - it's a good idea to have a large heavy pot lid on hand, just in case. Cook for a couple minutes (2-3 depending on how hot your pan is) on each side, until nicely blackened. Place pan into preheated oven and roast for about 5 minutes, or until flesh pulls apart easily with a fork. Chef's Notes: Be sure to monitor your pan carefully at all times, especially while you are preheating and working over open flame. You want to get the pan as hot as possible, but there is always a chance of fire when working with extremely high heat. NEVER LEAVE YOUR PAN UNATTENDED!!! This really isn't a dangerous technique as long as you are cautious and prepared. Have a heavy lid on hand to smother any possible flare-ups.