Translation:I found this in between the plates that were in the kitchen.
Just a note: it is really bookish (jež), it is used mainly in written communication.
I like knowing "bookish" Czech, as long as you tell us the difference. My real-life Czech teacher said to me, "Mluvíš jako kníha." I once translated "It was raining" as "Déšť padla." :)
I think both bookish and informal Czech should be teached, but unfortunately, Duolingo doesn't really allow to specify what is the difference, only in the pre-lesson texts.
I would just point out that "between" isn't the best word here. As a native English speaker, unless the item was literally standing between two plates, we would use either "among" (in, around or next to) or just say "I found this next to the plates in the kitchen."
You've got to put it from the Czech perspective here, though. The word "mezi" would still be used even if that thing were literally stuck between two plates. On the other hand, the next-to-the-plates location would very often be expressed as "našel jsem to u talířů", or "vedle talířů"… countless options; where should it end then? :-)