Preheat grill to medium-high heat.
In a small saucepan, bring the balsamic vinegar to a boil. Lower the heat and reduce the vinegar to about 1/3 cup, and set aside.
In a medium saucepan with boiling water, add the pearl onions, and cook until soft but still firm, about 4 minutes. Drain and let stand until cool enough to handle. Remove the skins, leaving the root intact and the onion whole.
In a large bowl, toss the swordfish cubes, onions, and olive oil together. Season lightly with salt and pepper and toss again.
Thread the fish, onions, and bay leaves onto wooden or metal skewers. Divide the ingredients evenly, alternating them on the skewers. Grill the fish skewers for about 1 minute on each side, for a total 4 minutes.
To serve, drizzle with a little of the balsamic reduction.