An ebi burger (which is made of prawns - it isn't a burger with a prawn on top of it) seems just as authentically Japanese to me as ebi gratin. It's a fine example of modern 'yoshoku' - as joejones says, it's not like you'll find it anywhere else.
I don't see how anyone can find pizza topped with nori or natto or any other of the bizarre combinations available in Japan to be comforting and familiar - those options are for travellers way more adventurous than I am, a lunchtime kaiseki-style menu is way less challenging!
Yoshoku is a very important part of Japanese cuisine and one's understanding of the food and culture suffers tremendously by ignoring it.
(Not perfect, but this article will familiarise you with the term if you're unaware of it)

As for the other foods I mentioned

The perhaps sad truth is that with the Alvalle gazpacho available from McDonalds, visitors do actually get a taste of what many Spaniards now think of as authentic Gazpacho Andaluz.
In two recent unrelated internet discussions I've seen fellow Spaniards recommend this exact brand to English speakers saying it was "just like their mother's" or "better than that served by many restaurants". These same people may not be aware that it's the brand served by MacDonalds... but I am.

And Quorn is a very popular food in the UK, but it would be completely natural for most visitors to remain completely unaware of its existence. A moot point now, as it isn't served by McD's anymore this is a genuine local food, designed for the demands of the local culture, that few tourists will get a chance to try.