Japanese Oshizushi: Pressed Sushi and the Rectangular Tradition

Ingredients

• For the sushi rice
• 300 g Japanese short-grain rice, washed and cooked
• 60 ml rice vinegar, slightly less than standard nigiri proportion
• 1 tbsp sugar
• 1 tsp salt
• For the battera topping
• 2 saba (mackerel) fillets, vinegar-cured
• 1 sheet kombu, cut to fit the oshibako base
• Equipment
• 1 oshibako (wooden pressing mould), cypress or cedar; three components: base, insert frame, lid

Method

1. 1
   Wet all interior surfaces of the oshibako with cold water. Do not oil. The wood should feel damp to the touch but carry no standing water; this prevents sticking and preserves the faint resinous aroma of the cypress or cedar.
2. 2
   Season the cooked rice with the vinegar, sugar, and salt, folding gently with a wooden paddle. The rice must cool to body temperature before pressing; it should feel barely warm at the centre of a cupped palm. Rice that is too hot will generate steam inside the mould, warping the wood and compromising compression.
3. 3
   Lay the kombu sheet face-down in the base of the oshibako, then place the vinegar-cured mackerel fillets skin-side down on top of the kombu. The fish surface must lie flat and cover the base evenly; any gap will show as a structural fault in the finished block.
4. 4
   Pack the seasoned rice evenly over the mackerel, pressing it down firmly with wetted fingers to fill the frame without air pockets. Fit the lid and press with consistent downward weight — a heavy cooking weight or book placed on the lid — for a minimum of ten minutes. This is where the dish lives or dies: insufficient compression will cause the rice to fragment on slicing, destroying both the structural integrity and the clean rectangular presentation that defines oshizushi.
5. 5
   Remove the weight and allow the pressed assembly to rest for twenty minutes. During this rest the kombu softens against the mackerel skin, releasing sweetness and deepening the umami of the block. The kombu sheet should feel pliable and slightly translucent at its edges when the rest is complete.
6. 6
   Invert the oshibako onto a clean board and release the frame. The block emerges with the kombu-mackerel topping uppermost and a clean face of rice beneath. Slice with a wetted knife into rectangular pieces. The cut surface should show distinct, cohesive layers of rice and fish with no crumbling at the edges.