It's characterized by its flavoured rice which is pressed into a box with topping of cooked or cured ingredients (either eel, red snapper, omelet, shrimp, or, the most famous, mackerel aka battera or “portugese sushi”). When it is pushed out of its wooden mould it gives a perfectly square block of rice (or rectangular in the case of the battera) which is then cut into smaller square pieces. It literally looks like Rice Krispies squares with toppings!
It seems easier to make than regular sushi (mostly rolls or vinegared rice pieces shaped into the palm) which usually required a long apprenticeship. Also, since it has no raw meat (sashimi) like the Edo-style sushi, it is safer to eat and requires no dip sauce.
I found two receipes: one on Food Network and one on Yummly. I even found a how-to video for battera on YouTube:
Unfortunately, after a quick search, I couldn't find a Japanese restaurant in Montreal that serves that type of sushi (if you find one, let me know!). I guess it only leaves us to try making it ourselves!