Kashubian Herring with Onions and Tomato-Raisin Sauce

Why You’ll Love This Recipe

• Classic Kashubian balance of sweet, sour, and savory with silky onions and gently tangy tomato.
• Make-ahead friendly: improves after a night in the fridge.
• Packed with pantry staples—great for entertaining without fuss.
• Elegant on rye for starters, canapés, or a light lunch.

Grocery List

• Produce: Yellow onions, fresh dill or chives, lemon (optional for serving)
• Dairy: Unsalted butter (for bread; optional)
• Pantry: Salt-cured herring or matjes, raisins, tomato paste, tomato passata, neutral oil, apple cider vinegar, sugar, allspice berries, bay leaves, mustard seeds (optional), black pepper, fine sea salt, dark rye bread

Full Ingredients

Herring

• 500 g salt-cured herring fillets (skinless if possible), or matjes fillets
• Cold water for soaking (if using salt-cured)
• Optional: 1 cup (240 ml) milk for a final 30-minute soak if herring is very salty

Onion-Tomato Sauce

• 3 medium yellow onions (about 600 g), halved and thinly sliced
• 3 tbsp neutral oil (sunflower or canola)
• 3 tbsp tomato paste
• 1 cup (240 ml) tomato passata (tomato purée)
• 1/2 cup (120 ml) water
• 3 tbsp apple cider vinegar (5% acidity)
• 2 tbsp sugar
• 1/2 cup (75 g) raisins
• 6 allspice berries, lightly crushed
• 2 bay leaves
• 1/2 tsp freshly ground black pepper, plus more to taste
• 1/4–1/2 tsp fine sea salt, to taste
• Optional: 1/2 tsp mustard seeds

To Serve

• 12 slices dark rye bread (about 1 cm thick)
• 2 tbsp unsalted butter, softened (optional)
• 2 tbsp chopped fresh dill or chives
• Optional: lemon wedges or small gherkins

Step-by-Step Instructions

Step 1: Choose and prepare the herring

If using salt-cured herring, rinse under cold water, then soak in a bowl of cold water 4–8 hours in the refrigerator, changing the water twice. If still very salty, give it a final 30-minute soak in milk, then rinse and pat dry. For matjes (pre-desalted), simply pat dry.

Cut fillets crosswise into bite-size pieces, about 2–3 cm wide. Set aside in the refrigerator.

Step 2: Plump the raisins

Place raisins in a heatproof cup and cover with just-boiled water. Let stand 10 minutes to plump, then drain thoroughly and set aside.

Step 3: Slow-sauté the onions

Warm the oil in a wide skillet over medium-low heat. Add onions and a pinch of salt. Cook, stirring occasionally, until very soft, translucent, and pale golden with sweet edges, 20–25 minutes. Avoid browning—this keeps the sauce gentle and mellow.

Step 4: Build the Kashubian tomato base

Push onions aside, add tomato paste to the center, and cook 1 minute to caramelize. Stir in passata, water, vinegar, sugar, allspice, bay leaves, black pepper, and optional mustard seeds. Bring to a gentle simmer and cook 10–12 minutes, stirring, until glossy and slightly thickened.

Step 5: Finish the sauce and cool

Fold in the drained raisins and simmer 1 minute more. Taste and adjust: add a pinch of salt if needed, a teaspoon of sugar for softness, or a splash of vinegar for brightness. Remove from heat, discard bay leaves, and cool to room temperature, about 15–20 minutes.

Step 6: Layer and marinate

In a clean glass or ceramic dish/jar, add a spoonful of onion-tomato sauce. Top with a layer of herring. Continue layering, finishing with sauce so the fish is fully covered. Cover tightly and refrigerate 12–24 hours (48 hours yields deeper flavor).

Step 7: Serve on rye

Toast rye slices at 180°C/350°F for 5–7 minutes if you like, then lightly butter. Spoon herring with plenty of onions and sauce onto the rye. Garnish with dill or chives and a crack of black pepper. Serve cool, not icy—let it sit 10 minutes at room temperature for best flavor.

Pro Tips

• Keep it gentle: slowly sauté onions to pale gold, not brown, for a sweeter, silkier sauce.
• Use non-reactive containers (glass or ceramic) to prevent metallic flavors in the vinegar-tomato sauce.
• Balance is key: aim for a softly tart sauce. Add sugar or vinegar in 1/2 teaspoon increments to fine-tune.
• Ratio guide: for every 1 part herring, use 1.5–2 parts onion-tomato sauce so the fish stays juicy.
• Bring to serving temperature: 10–15 minutes out of the fridge unlocks aroma and texture.

Variations

• Prunes and smoked note: Replace half the raisins with chopped prunes and add a pinch (1/8 tsp) of smoked paprika.
• Chili warmth: Stir in 1/4–1/2 tsp red pepper flakes with the tomato paste for a gentle heat that loves rye.
• Citrus-bright: Add 1 tsp finely grated orange zest to the cooled sauce before layering for a fragrant twist.

Storage & Make-Ahead

Refrigerate in a sealed glass container for up to 5 days; flavor improves after 24–48 hours. Always keep the herring submerged in sauce. For best texture, assemble the rye toasts just before serving. Do not freeze (the fish becomes mealy). Use clean utensils each time to extend shelf life.

Nutrition (per serving)

Approximate values for 1 of 6 servings with rye and butter: 580 calories; 27 g fat; 55 g carbohydrates; 23 g protein; fiber 5 g; sodium varies depending on herring desalting.