Preparation 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 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. Preheat oven to 250°F. Bring relish to room temperature. 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. Transfer fish to a platter. Mix parsley into relish and spoon over fish. Do Ahead Relish (without parsley) can be made 1 day ahead. Keep chilled.
Preparation 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 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. Preheat oven to 250°F. Bring relish to room temperature. 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. Transfer fish to a platter. Mix parsley into relish and spoon over fish. Do Ahead Relish (without parsley) can be made 1 day ahead. Keep chilled.