I'm a bit confused about this sentence because Imbissstände is plural, but einen is singular accusative and I'm not sure if der is plural genitive or masculine nominative.
3 Answers
"Einen" is accusative singular
"der Imbissstände" is genitive plural
Ihr seht einen Imbissstand aus der Gruppe der berühmten Imbissstände
That is, one out of many.
┏━━ADVERBIAL━━┓ ┏━PREDICATE━┓ ┏━SUBJECT━┓ ┏━━━━━━━━━ACCUSATIVE OBJECT SINGULAR━━━━━━━━━━┓ Hinter mir seht ihr einen der berühmtesten Imbissstände. └──────GENITIVE PLURAL NOUN PHRASE─────┘
Behind me you're seeing one of the most famous snack stalls.
Why do you assume that both should have the same case? Einen is in accusative singular, while der in genitive. Notice that der cannot be nominative, because the noun is in plural (Imbissstände).