Transfer the foil packets to the grill away from hot coals, cover the grill, and cook for 15 minutes. Pinch the ends closed, and fold once or twice to seal tightly. Bringing two sides of the foil together above the fish, seal and roll down close to the food. Top with one seasoned bluefish fillet, and spread equal portions of the shallot-caper mixture over each fillet. On the middle of each double-layered square, arrange equal amounts of tomato and onion, and drizzle with remaining marinade. Cut 12 pieces of aluminum foil, approximately 15 inches square, to make six 2-layer packets. Mix the soy sauce and lemon juice in a small bowl, and brush each fillet generously on both sides with the mixture. Add the dill and parsley, and pulse a few times to combine. (May be prepared in advance up to this point cover and refrigerate in the marinade overnight, allowing them to return to room temperature before serving.) Maintain the grill heat for the fish fillets.Ĭombine in a food processor the shallots, capers, mustard, peppercorns, and horseradish, and process until puréed. Return the slices to the marinade, and set aside until ready to serve. Place the slices over the hot coals and cook until charred (tomatoes and onions will char in the dry pan as well). Remove tomato and onion from the marinade, allowing excess oil to drip off, reserving marinade.
Season with salt and pepper and drizzle with the olive oil. Arrange the onion and tomato slices in one layer on a cookie sheet or in a roasting pan, and sprinkle with garlic and thyme. Prepare a medium-hot grill (if cooking indoors, preheat a cast-iron sauté pan until it is very hot).