In a blender, combine the onions and lemon juice and purée until very smooth.
Cut the swordfish into 1-oz/28-g pieces, season with salt and pepper, and combine with the lemon juice–onion mix. Add the olive oil and allow to marinate for up to 1 hour.
Skewer 6 oz/170 g swordfish onto each flat metal skewer with a bay leaf between each piece.