Any white-flesh fish will readily take on bolder flavors, such as those in this rich balsamic mixture brightened by fresh red onion, salted capers, aromatic parsley, and pungent anchovies.
Whisk together all of the ingredients in a medium nonreactive bowl. Use marinade immediately.
This marinade works best with 1-inch-thick halibut or other firm, white-flesh fish fillets. Marinate the fish for 2 hours, and then bake in and baste with the marinade in a 350-degree oven.