2 tablespoons olive oil
3 pounds swordfish steaks — skin removed, cut into 1-inch cubes
1 1/2 teaspoons kosher salt
1/2 teaspoons freshly ground pepper
1/2 cup (1 stick) unsalted butter
1/4 cup balsamic vinegar
2 teaspoons Dijon mustard
2 tablespoons fresh chives — additional for garnish

To make the marinade, place the olive oil, swordfish, salt and pepper in
a nonreactive bowl and mix until well coated. Cover and refrigerate 3 to 4
hours. While the fish marinates, place butter in saucepan over low heat
and cook. Just as it begins to foam and is well browned, about 5
minutes, add the balsamic vinegar and cook until it thickens, about 3
minutes. Add mustard and chives and cook additional 2 minutes. Set
aside. Prepare grill. If using charcoal, when coals are glowing red,
after 15 to 20 minutes, cover with a grate. After 5 minutes, use a wire
brush to clean the grate. When coals are covered with a pale gray ash
and you can leave your hand 5 inches above the fire for 2 to 3 seconds,
it’s ready. Thread swordfish on skewers and grill, turning every 1 1/2
minutes until swordfish is well browned, about 7 to 8 minutes total.
Transfer to serving platter and drizzle with sauce. Garnish with
additional chives.