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Cod with Lemon, Green Olive, and Onion Relish

Peer reviewed by UK recipe editorsAuthored by UK recipe editorsOriginally published 16 Jan 2026

Meets Patient’s editorial guidelines

This Mediterranean-inspired cod with lemon, green olive, and onion relish is a vibrant and nutritious choice for a light supper. By slow-roasting the cod fillets in olive oil at a low temperature, the fish remains incredibly moist and buttery, providing a beautiful contrast to the piquant, citrus-forward relish. The combination of briny Castelvetrano olives, sharp capers, and softened red onions creates a sophisticated flavour profile that elevates the delicate white fish without overpowering it.

As a diabetes-friendly dish, this recipe focuses on high-quality fats and lean protein, making it a heart-healthy addition to your weekly menu. The relish can be prepared a day in advance, allowing the flavours to develop and making it an excellent option for stress-free entertaining. Serve it alongside steamed seasonal greens or a crisp garden salad for a balanced, low-carbohydrate meal that feels truly indulgent.

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Ingredients for Cod with Lemon, Green Olive, and Onion Relish

  • 2 lemons

  • 1/2 small red onion, very thinly sliced into rings

  • 1 teaspoon kosher salt plus more

  • 120ml Castelvetrano or other brined green olives (about 24), coarsely chopped

  • 2 tablespoons drained capers, chopped

  • 300ml olive oil, divided

  • Kosher salt, freshly ground pepper

  • 8 (170g) pieces skinless cod fillets

  • 2 dried red chillies, stemmed, or 1/4 teaspoons crushed red pepper flakes

  • 1/4 cup fresh flat-leaf parsley leaves

How to make Cod with Lemon, Green Olive, and Onion Relish

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 180ml oil; season with salt and pepper and toss to combine. Cover relish and chill at least 4 hours.

Preheat oven to 121°C. Bring relish to room temperature.

Place cod in a large shallow baking dish or roasting pan and drizzle with remaining 120ml oil. Add chillies 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.

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

Disclaimer

While every effort has been made to ensure the information is accurate and up to date, individual needs may vary and dietary requirements can differ based on personal health conditions. Always check food labels and allergen information before preparing or consuming any recipe. If you have specific health concerns, allergies, intolerances, or are following a medically prescribed diet, seek advice from your GP, pharmacist, or a registered dietitian before making significant changes to your diet or lifestyle.

Article history

The information on this page is peer reviewed by qualified clinicians.

  • 16 Jan 2026 | Originally published

    Authored by:

    UK recipe editors

    Peer reviewed by

    UK recipe editors
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