Slice the onion very thinly and put into a bowl of water. Squeeze out well and mix with mayonnaise. This will be used as a topping and is optional.
Cut each piece of cheese into 12 pieces (about 8 mm dice). Finely chop the shiso leaves. Reserve a bit of each to use as topping.
Line a deep plate (such as a soup plate) with plastic wrap, and line with half the amount of smoked salmon (1 serving).
Mix the ponzu sauce with hot rice, then add the non-reserved cheese and shiso leaves.
Mound half the mixed rice (1 serving) on top of the salmon in the plate.
Wrap with the plastic wrap and press lightly to form a round shape.
Open the plastic wrap and invert onto a plate. Top with the onion and mayonnaise, bonito flakes and reserved shiso leaves, then scatter the reserved cheese around the perimeter.
Make another portion just like the first and it's done.
Story Behind this Recipe
I don't stock readymade sushi vinegar in our kitchen. I tried using handy ponzu sauce instead of sushi vinegar, and it was very refreshing and delicious. It went really well with the rich flavor of cheese, so I decided I wanted to share the recipe.
You can use sashimi grade tuna instead of the smoked salmon. You can also serve this sushi wrapped in a thin omelette (usuyaki tamago) like a rice omelette.