Preheat oven to 350 degrees.
Sprinkle fish fillets with salt and pepper. Place a large cast-iron or other heavy-bottomed frying pan over medium-high heat. When hot, add 3 tablespoons of the olive oil. When the oil is hot, add the fish fillets and sear, turning once, until lightly golden on both sides, 1-2 minutes per side. Transfer the fillets to an ovenproof platter or shallow baking dish and place in the oven to finish cooking until opaque throughout, 3-5 minutes depending upon thickness of fillets.
Meanwhile, add the remaining 5 tablespoons olive oil to the same pan and place over medium-low heat. Add garlic, 1 teaspoon salt, and ½ teaspoon black pepper. Saute for 1-2 minutes. Add the ancho chili and continue to saute until fragrant, 1-2 minutes longer. Add the fish stock or clam juice and any juices that have collected in the platter or dish holding the fish fillets and raise the heat to medium-high. Boil until reduced by half, 1-2 minutes. Add the lime juice and parsley and bring to a boil, then remove from heat.
Pour the sauce over the fish fillets on the platter. Or, if using a baking dish, transfer the fillets to a warmed platter and pour the sauce over the top. Serve immediately.
Garlic sauce, known as mojo de ajo, is a favorite topping for Mexican seafood. This version adds a touch of ancho chili. Make the sauce quickly while the fish finishes cooking, and take care not to brown the garlic or chilies. Serve with red or white rice.