Saba Oshi-Zushi Osaka Pressed Sushi Tradition

Ingredients

• For the shimesaba (vinegar-cured mackerel)
• 1 whole fresh mackerel (saba), filleted, pin-boned, skin on
• 30 g fine salt, for curing the fillet
• 120 ml rice vinegar, for marinating the cured fillet
• For the sushi rice
• 300 g Japanese short-grain rice, washed until water runs clear
• 360 ml water
• 60 ml rice vinegar, slightly increased proportion for structural acidity
• 20 g caster sugar
• 8 g fine salt
• For assembly
• 1 sheet kombu, thin sheet, cut to fit mould dimensions
• 1 length plastic film, for lining the oshibako mould

Method

1. 1
   Salt the mackerel fillet generously on both sides and lay it flat on a tray. Leave for 30 minutes so the salt draws moisture from within the flesh outward, visibly firming the surface and leaving it wet to the touch. Do not rush this stage — the pellicle that forms is what the vinegar will then work against.
2. 2
   Rinse the salt from the fillet under cold running water and pat it completely dry, so the vinegar is not diluted when the fillet goes in. Submerge the fillet in rice vinegar in a flat dish and leave for 30 minutes to 1 hour, watching for the flesh to convert from translucent-raw to an opaque, silver-white all the way through to the centre — that full opacity is your sign to stop. Remove and pat dry; the shimesaba is ready.
3. 3
   Cook the washed rice with the measured water. Dissolve the vinegar, sugar, and salt together over low heat without boiling, so the sugar and salt melt cleanly into the vinegar without any caramelisation altering the balance. Turn the cooked rice out into a wide non-reactive bowl, fold the seasoned vinegar through with a cutting motion and fan as you go, then allow it to cool fully — warm rice sealed into the mould will generate steam that collapses the structure you are about to build, so do not proceed until the rice is cool to the touch.
4. 4
   Line the oshibako mould completely with plastic film, allowing overhang on all sides so the block can be lifted free cleanly. Lay the shimesaba fillet skin-side down across the base in a single even layer, trimming or folding the tail end to fill the rectangle without gaps, then lay the kombu sheet evenly over the fish. This is where the dish lives or dies: the rice must be distributed in an absolutely even layer over the fish — any uneven compression will fracture the block on unmoulding and shatter the clean line of silver skin over white rice.
5. 5
   Pack the seasoned rice over the kombu in an even layer, pressing gently by hand to close any air pockets before the lid goes on, so the block does not crack or hollow when cut. Fold the overhanging plastic film over the rice surface, replace the mould lid, and press with a heavy weight — a heavy tin or similar object — for a minimum of 30 minutes and up to 2 hours. At 2 hours the rice and fish will have merged into a single cohesive block; the surface will not spring back under thumb pressure.
6. 6
   Remove the weight and lid, then invert the mould onto a cutting board and lift it away before peeling back the plastic film, so the silver skin is uppermost and unmarked. Rest the unmoulded block for a minimum of 30 minutes — or refrigerate overnight — before cutting, because the resting allows the layers to settle and bond further; unlike nigiri, the pressed block improves with time, and overnight refrigeration deepens the integration of the cured fish and seasoned rice.
7. 7
   Wet a sharp knife and slice the block into eight equal rectangular portions using a single downward stroke per cut, wiping and re-wetting the blade between each one so starch does not drag. A sawing motion tears the rice structure; the cut face must show a clean, unbroken boundary between the white rice and the silver-skinned fish above.