Ingredients

• 2 lemons
• 1/2 small red onion, very thinly sliced into rings
• 1 teaspoon kosher salt plus more
• 1/2 cup Castelvetrano or other brined green olives (about 24), coarsely chopped
• 2 tablespoons drained capers, chopped
• 1 1/4 cups olive oil, divided
• Kosher salt, freshly ground pepper
• 8 6-ounce pieces skinless cod fillets
• 2 dried red chiles, stemmed, or 1/4 teaspoons crushed red pepper flakes
• 1/4 cup fresh flat-leaf parsley leaves

Instructions

1. 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.
2. Combine onion and 1 teaspoon 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 3/4 cup oil; season with salt and pepper and toss to combine. Cover relish and chill at least 4 hours.
3. Preheat oven to 250°F. Bring relish to room temperature.
4. Place cod in a large shallow baking dish or roasting pan and drizzle with remaining 1/2 cup oil. Add chiles and turn fish to coat; season with salt. Roast until just cooked through, 30–40 minutes.
5. Transfer fish to a platter. Mix parsley into relish and spoon over fish.
6. DO AHEAD: Relish (without parsley) can be made 1 day ahead. Keep chilled.