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.