There was a time period in the 1950s where Americans became obsessed with Hawaii vacations (likely due to the affordability of air travel) and companies started

There was a time period in the 1950s where Americans became obsessed with Hawaii vacations (likely due to the affordability of air travel) and companies started using Hawaii themes in all kinds of branding. I wouldn't be surprised if Toast Hawaii was spillover from that (as vacationing in Hawaii was much less affordable to Germans and thus most exposure to it was second-hand from US media portrayals). I'd also not be surprised if the association with pineapple and cheese somehow made it back to the US (possibly by proxy, e.g. via European immigrants to North America) only to be "invented" by some Canadian guy. Given that a lot of origin stories tend to be created after the fact (e.g. infamously the board game Monopoly's origin story is largely fabricated as it was obviously based on an existing game already widely referred to as "the monopoly game" which in turn was originally based on "The Landlord's Game") I'm cautious about direct attributions without clear evidence. It's entirely possible that "baked pineapple and cheese" already existed as a niche topping at the time the first restaurant offically added it to their menu. Interestingly Toast Hawaii may have been inspired by the American "grilled spamwich", which also included pineapple and (shredded) cheese but instead of cooked ham used spam, which wasn't available in Germany at the time.