Packaged shime saba is widely available in Japan and it is enjoyed as sashimi or sushi. Alternatively, you can use grilled mackerel seasoned with salt or smoked salmon.
Searing the skin of shime saba is optional but the nice and crispy outside will definitely stimulate your appetite.
If the shiso leaves are not available, you can use cucumber instead. Let salted cucumber slices sit for a while and tightly squeeze out the excess liquid before adding.