Japanese Saba Oshizushi: Mackerel Pressed Sushi and the Battera Osaka Tradition

Ingredients

• For the saba (mackerel)
• 1 (approx. 400 g whole, yielding 2 fillets) Pacific mackerel (saba), pin-boned, skin on; autumn mackerel preferred for fat content
• 30 g fine sea salt, for the initial dry cure
• 200 ml rice vinegar, for the vinegar cure bath; enough to submerge fillets
• For the shari (seasoned sushi rice)
• 300 g short-grain Japanese sushi rice, uncooked weight; rinsed until water runs clear
• 60 ml rice vinegar, for seasoning the cooked rice
• 15 g white sugar, dissolved into the shari seasoning
• 8 g fine sea salt, dissolved into the shari seasoning
• For the pressing
• 2–3 sheets shima konbu (striped dried kelp), thin sheets; sized to cover the top of the oshibako
• 1 oshibako (wooden sushi pressing mold), soaked in water before use if unseasoned; or a loaf tin lined with plastic wrap

Method

1. 1
   Lay the mackerel fillets skin-side down on a tray. Distribute the fine sea salt evenly and generously over the flesh surfaces. Leave uncovered at room temperature for 1 hour. The salt draws visible moisture from the flesh, firming the fillet noticeably to the touch — this structural change confirms sufficient salt curing before proceeding.
2. 2
   Rinse the salt from the fillets under cold running water. Pat fully dry with kitchen paper. Lay the fillets in a flat, non-reactive dish and pour the rice vinegar over them, ensuring complete submersion. Cure for 2 to 3 hours. The dish lives or dies here: the skin surface must turn silvery-white and semi-opaque — a visual denaturation that reaches the skin surface but does not penetrate the full depth of the fillet. A fillet white all the way through is over-cured; a fillet still pink beneath the skin surface is under-cured. Remove at the point of correct opacity.
3. 3
   While the mackerel cures, cook the sushi rice according to its absorption ratio. Dissolve the sugar and salt into the rice vinegar over gentle heat, then fold the seasoning through the hot cooked rice with a cutting motion, fanning to cool. The shari is ready when it is glossy, slightly sticky, and has cooled to room temperature — individual grains hold together when pressed but do not crush to paste.
4. 4
   If using an unseasoned wooden oshibako, soak it in cold water for several minutes before use; dry wood will draw moisture from the shari and alter its texture. Line the mold (or loaf tin) with plastic wrap, leaving sufficient overhang on all sides. Lay the shima konbu sheets in a single layer across the base of the mold.
5. 5
   Remove the cured mackerel from the vinegar. Pat dry. Lay the fillets skin-side down in an even layer over the shima konbu, trimming and fitting to cover the mold base without gaps. The skin-side — now silver-white and semi-opaque — faces the kelp; the flesh faces upward.
6. 6
   Pack the seasoned shari firmly and evenly over the mackerel layer, pressing it into an even depth throughout the mold. Fold the plastic wrap overhang over the top. Set the oshibako lid (or a cutting board cut to fit the tin) over the wrapped surface and apply firm, even weight. Press for a minimum of 30 minutes; 2 hours produces a more unified block, and overnight pressing at room temperature below 20°C produces the most fully integrated result. The block is ready when it holds its shape cleanly upon unmolding and the layers do not separate under gentle lateral pressure.
7. 7
   Unmold the battera onto a board, kelp-side up. With a wet knife and firm downward pressure — no sawing — cut the block into even rectangular pieces approximately 3 cm wide. Wipe and re-wet the blade between cuts. Each piece should present the clean three-layer cross-section of konbu, white mackerel skin, and shari. Serve at room temperature; the rice must not be refrigerated cold, as temperatures below 18°C harden the shari and mute the mackerel's flavor.