• 0.5 small red onion, very thinly sliced into rings
• 1 teaspoon kosher salt plus more
• 0.5 cup Castelvetrano or other brined green olives (about 24), coarsely chopped
• 2 tablespoons drained capers, chopped
• 1.25 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
• 0.25 cup fresh flat-leaf parsley leaves
Steps
Step 1Finely 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.
Step 2Combine 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.
Step 3Preheat oven to 250°. Bring relish to room temperature.
Step 4Place 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.
Step 5Transfer fish to a platter. Mix parsley into relish and spoon over fish.
Step 6DO AHEAD: Relish (without parsley) can be made 1 day ahead. Keep chilled.
