It's an adjective. For this word, we have:
- ενδιαφέρων, masculine
- ενδιαφέρουσα, feminine
- ενδιαφέρον, neuter
There aren't many adjectives like this in the course (I think this might be the only one?) but these sort of endings do exist in modern Greek. Words like this are either largely unchanged from their Ancient Greek form, or they were constructed to 'look' Ancient during the Greek language reforms of the last century.
In this sentence, ενδιαφέρον is in nominative, because είναι doesn't trigger accusative, but I'm guessing you only asked which case it was in because the adjective looked 'unusual'?
Just to complicate matters, ενδιαφέρον is also a neuter noun for 'interest', and this does appear elsewhere in the course. So, we can also say something έχει ενδιαφέρον - has interest - if we want to say it's interesting.
'Interest' as a noun is always neuter: ενδιαφέρον.
The adjective 'interesting' takes the gender of the noun it accompanies, here φαγητό is neuter too, so ενδιαφέρον. Ενδιαφέρων is the masculine form and ενδιαφέρουσα the feminine one (e.g. ενδιαφέρων αγώνας, ενδιαφέρουσα προσωπικότητα).