Prepare the grill by lighting 4 quarts of charcoal (1 starter chimney's worth), or turning gas grill to medium-high.
Examine the steaks for pin bones by rubbing fingers lightly over the surface of the meat and using a clean pair of needle-nose pliersMake sure you pull with the grain of the meat. to remove. Using a sharp paring or boning knife, trim the backbone from the inside of the steak, then trim the stomach flaps of each steak so that 1 side is missing about 2 inches of skin, and the other, 1 inch of meat. Roll the skinless section up into the hollow cavity, then wrap the flap of skin around the outside to form a round resembling a filet mignon. Tie in place with two passes of butcher's twine. Do not tie it too tight or the fish will pop out during cooking.
Combine the cumin, coriander, fennel, and peppercorns on a double-thick piece of aluminum foil and toast over the charcoal starter, shaking gently with tongs until the seeds become fragrant. Cool briefly, then coarsely grind in a blade-style electric coffee grinder. Coat steaks lightly with oil, season with a sprinkling of salt, then liberally grind toasted seeds on both sides of steaks.
When the charcoal is completely lit, move from the starter to the grill grate. Plate the cooking grate over for several minutes to heat, then quickly wipe with a rolled dish towel or paper towels (held with tongs) dipped in a bit of cooking oil.
Grill the fish to medium-rare, about 3 minutes per side. Keep the grill lid closed and do not move the fish more than once during the process. The fish should be well colored on the outside and barely translucent at the center.