Cod with Lemon, Green Olive and Onion Relish

This slow-roasted cod topped with zesty relish is just the thing for an end-of-winter dinner party.

2 lemons
½ small red onion, very thinly sliced into rings
1 tsp. kosher salt
½ cup Castelvetrano or other brined green olives (about 24), coarsely chopped
2 Tbsp. capers, drained, chopped
1¼ cups olive oil, divided
kosher salt
black pepper, freshly ground
8 (6 oz) pieces cod fillets, skinless
2 dried red chiles, stemmed, or ¼ tsp. crushed red pepper flakes
¼ cup fresh flat-leaf parsley leaves

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°F (120°C/100°C fan, Gas Mark 0.5). 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 until just cooked through, 30-40 minutes.

Transfer fish to a platter. Mix parsley into relish and spoon over fish.

Makes 8 servings.

Make Ahead: Relish (without parsley) can be made 1 day ahead. Keep chilled.