Preheat your grill to medium heat. Preheat a skillet or grill basket with small holes on the grill for the relish. Something that the pieces will not fall through.
Season both sides of the fish with salt and pepper. Brush your grill (and/or the fish, honestly) with some oil to help prevent it from sticking.
Meanwhile, prepare your grapefruit salsa. Cut the top, bottom, and skin off of the grapefruit and then slice your knife right next to each segment to remove them, leaving you with just the flesh wedges (see picture).
Cut the grapefruit wedges that you have removed into chunks (a little larger than you want, as they will break some with mixing) and place in a small bowl. Squeeze the "guts" (leftover white membrane from the grapefruit) of one grapefruit into the small bowl to get some of the liquid.
Add the finely diced red onion, basil, juice of 1/2 lemon, and the olive oil to the grapefruit. Stir gently to combine. Season to taste with salt and pepper. If too tart, add a little sugar as desired.
When the grill is hot, add the swordfish to the grill.
Cook the fish for approximately 5 minutes. The fish should release somewhat easily from the grill. Flip over.
Finish cooking the fish, an additional 5 minutes or so on the second side. Remove from the heat.
Top the fish with the relish, or serve on the side. Serve immediately.
This recipe is great served on top of a rice or vegetable like asparagus. Double up the relish recipe if you want a lot of it!I try to remove as much of the white part of the grapefruit as possible because it can be bitter, but a little here and there is fine.Mint is a great substitute for the basil, and orange is a great substitute for the grapefruit.