½ cup Castelvetrano or other brined green olives (about 24), coarsely chopped
2 tablespoons drained capers, chopped
1¼ cups olive oil, divided
Freshly ground black pepper
8 6-oz. pieces skinless cod fillets
2 dried red chiles, stemmed, or ¼ tsp. crushed red pepper flakes
¼ cup fresh flat-leaf parsley leaves
Instructions
Finely grate zest from lemons; set aside. Using a sharp knife, cut peel and white pith from lemons; discard. Working over a medium bowl, cut between membranes to release segments into bowl and squeeze membranes to release juices; discard membranes and any seeds. Set lemon segments aside.
Combine onion and 1 tsp. salt in a small bowl. Let sit 10 minutes. Squeeze onion to remove any excess liquid; add to bowl with lemon segments. Add olives, capers, reserved lemon zest, and ¾ cup oil; season with salt and pepper and toss to combine. Cover relish and chill at least 4 hours.
Preheat oven to 250°. Bring relish to room temperature.
Place cod in a large shallow baking dish or roasting pan and drizzle with remaining ½ cup oil. Add chiles and turn fish to coat; season with salt. Roast fish until just cooked through, 30–40 minutes.
Transfer fish to a platter. Mix parsley into relish and spoon over fish.