Method

1. Make the garum. Combine the oregano, thyme, olives, anchovies, and bread in a food processor and blend until the mixture froms a paste. Slowly drizzle in the oil until the ingredients are incorporated and the mixture is smooth. Set aside.
2. Make the sauce. Heat 2 tablespoons of the butter in a medium saucepan over medium heat and add theonions. Sauté, stirring, frequently, until tender, 3 to 5 minutes, and add the sofrito. Stir together for a minute and whisk in the fish stock. Turn the heat to high and bring to a boil, then reduce the heat to medium high and continue to cook until the volume is reduced to about 1 cup, 18 to 20 minutes. Stir continuously during the last 5 to 10 minutes to ensure that the sofrito vegetables do not stick to the bottom of the pan. Reduce the heat to medium low and whisk in the cream and the remaining butter. Set aside and keep warm.
3. Preheat a griddle or a gas charcoal grill to medium-high heat. If you are using a charcoal grill, build a fire with coals and let it burn down until the coals are glowing red with a thin coating of white ash. Spread the coals out in a single layer. Brush a grill grate with oil, place the grill grate on a regular grate and preheat for 5 minutes. Brush the halibut fillets with 2 tablespoons of oil, then sprinkle each with a pinch of salt. Place the fillets skin side down on the hot grill grate and grill for 3 minutes. Gently turn the fish with a spatula or tongs, and cook it for another 4 to 5 minutes on the other side, or until firm but not hard. If desired, check the inside of the thickest part of a fillet with a knife; the center should still be slightly translucent. Remove the skin from the fish, if desired, and transfer the fillets to a platter; they will continue to cook as they rest.
4. Spoon 2 ½ tablespoons of sauce over each plate. Set a fish fillet on top. Top the fish with about 1 tablespoon of the garum and a few strips of the crisp Serrano ham. Repeat with the remaining fish fillets. Drizzle ¼ teaspoon of oil around each plate.