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Flounder Poached in Fennel-Tomato Sauce

This poached flounder in fennel and tomato sauce is a sophisticated yet simple high-protein dish that brings the light, fresh flavours of the Mediterranean to your kitchen. By gently simmering the delicate white fish in a fragrant tomato base, you ensure the fillets remain moist and tender while absorbing the aromatic notes of aniseed from the fennel and tarragon. The addition of crushed red pepper flakes provides a subtle, warming heat that perfectly balances the sweetness of the softened fennel bulb.

Ideal for a nutritious midweek supper, this recipe is naturally low in fat and packed with lean protein. It is a wonderful way to introduce more seafood into your diet without the need for complex techniques or heavy sauces. For a complete and satisfying meal, serve the fish in shallow bowls with a side of steamed greens or a crusty piece of wholemeal bread to soak up the delicious savoury juices.

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Ingredients for Flounder Poached in Fennel-Tomato Sauce

  • 3/4 teaspoons fennel seeds

  • 1/8 teaspoons crushed red pepper flakes

  • 1 tablespoon olive oil

  • 1 small fennel bulb, thinly sliced, plus fennel fronds for garnish

  • Kosher salt, freshly ground pepper

  • 350ml store-bought marinara sauce, preferably with no sugar added

  • 110g pieces skinless flounder fillet

  • Coarsely chopped fresh tarragon

  • A spice mill (optional)

Coarsely grind fennel seeds and red pepper flakes in a spice mill or with a mortar and pestle. Alternatively, place in a resealable freezer bag. Crush with a rolling pin or large skillet. Set aside.

Heat oil in a large skillet over medium-low heat. Add sliced fennel, season with salt and pepper, and cook, stirring often, until soft, 10-12 minutes. Add spices and cook, stirring often, until spices are beginning to darken, about 1 minute.

Add marinara sauce and 120ml water to skillet; bring to a simmer and cook to meld flavours, about 5 minutes. Season with salt and pepper.

Season fish with salt and pepper and fold fillets in thirds or in half to form packets. Place fillets in sauce. Cover skillet and reduce heat to medium-low. Cook until fish is opaque and beginning to flake, 12-15 minutes.

Divide fennel-tomato sauce and fish among shallow bowls. Sprinkle tarragon and fennel fronds over.

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.

  • 28 Jan 2026 | Originally published

    Authored by:

    UK recipe editors

    Peer reviewed by

    UK recipe editors
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