Season the swordfish with sea salt and the Fennel Spice Rub, pressing the spices into the flesh. Tie the swordfish crosswise at 3 evenly spaced places (not imperative but it will hold together a little easier).
Heat the olive oil in a 12-inch skillet over medium heat. Brown the swordfish on all sides, turning occasionally, about 15 minutes total.
Place the swordfish on the rack inside the baking sheet and roast until the internal temperature reaches 120 degrees F, or about 30 minutes. Allow the fish to rest at room temperature for 10 minutes before carving.
Cut the strings to remove, and carve the swordfish crosswise into 1/2-inch slices.
Put the fennel seeds, coriander seeds, and peppercorns in a heavy pan over medium heat, tossing frequently so the seeds toast evenly. Watch carefully as they can easily burn.
When light brown and fragrant, pour the seeds onto a plate to cool. They must be cool before grinding, or they will gum up the blades.
Pour the seeds into a blender and add the salt. Blend to a fine powder, shaking the blender occasionally to redistribute the seeds. Store in a tightly sealed glass jar in a cool, dry place, or freeze.
