Remove fat cap off top of meat (butcher can do this for you) and save. Place the roast, standing on the rib bones, in a very large roasting pan. Then with a knife make several dozen punctures through the silver skin so seasoning can permeate meat.
Pour a very generous, even layer of black pepper over the top of the meat (the pepper should completely cover it); repeat with the garlic powder, then the salt, totally covering the preceding layer.
Carefully arrange the onions in an even layer on top so as not to knock off the seasoning.
Place the fat cap back on top. Refrigerate 24 hours.
Bake ribs in a 550F oven until the fat is dark brown and crispy on top, about 35 minutes. Remove from oven and cool slightly. Refrigerate until well chilled, about 3 hours. (this is done so the juices will solidify and the steaks can be cooked rare.)
Remove fat cap and discard.
With the blade of a large knife, scrape off the onions and as much of the seasonings as possible and discard. Then with a long knife, slice between ribs into 6 steaks (4 will have bones); trim the cooked surface of meat from the 2 pieces that were on the outside of the roast. Season and cook in your favorite way for steaks.
TO BLACKEN THE STEAKS: Combine the ingredients of the seasoning mix thoroughly in a small bowl; you will have about 8 tablespoons. Sprinkle the steaks generously and evenly on both sides with the mix. using about 4 teaspoons on each steak and pressing it in with your hands.
Heat a cast iron skillet over very high heat until it is beyond the smoking stage and you see white ash on the skillet bottom--at least 10 minutes. (The skillet cannot be too hot for this method.)
Place one steak in the hot skillet (cook only one side at a time) and cook over a very high heat until the underside starts to develop a heavy, black crust, about 2 to 3 minutes. Turn the steak over and cook until the underside is crusted like the first, about 2 to 3 minutes more. Repeat with the remaining steaks. Serve each steak while piping hot.
NOTE: If you don't have a commercial hood vent over your stove, this dish may smoke you out of the kitchen. It's worth it! But you can also cook it outdoors on a gas grill; a charcoal fire doesn't get hot enough to "blacken" the steak properly. If you have a smoke detector in your house, you will be able to determine if it is working correctly. This is NOT a dish to prepare in an apartment building with a central fire alarm system wired into your smoke detector. It causes great excitement! Also, you can be guaranteed you will meet your landlord.