Cook the sushi rice according to the package directions.

In a small bowl, combine the rice vinegar, sugar, and salt. Microwave for about 10 seconds, or until the sugar dissolves. Stir well.

While the rice is still warm, gently fold the vinegar mixture into the cooked rice. Fluff the rice and allow it to cool to room temperature.

Line a small round container or food storage container with plastic wrap, leaving enough overhang to help unmold the sushi cake later.

Arrange the sashimi slices in an even layer across the bottom of the container.

Add a layer of shiso leaves.

Spread a layer of sushi rice over the shiso leaves and gently press it down to create an even layer.

Arrange the cucumber slices over the rice.

Add another layer of sashimi.

Finish with another layer of sushi rice, pressing gently so the cake holds its shape.

Cover and refrigerate for at least 15 minutes to help the layers set.

When ready to serve, place a serving plate over the container. Carefully invert the container and gently lift it away, then remove the plastic wrap.

Garnish with an avocado rose, salmon roe, extra sashimi, sesame seeds, or additional shiso leaves before serving.