"A meat sandwich"
Translation:Kanapka z mięsem
1) we usually have sandwiches with something in Poland
2) if anything "meat" should be an adjective here as the question is not czego? ( of what) but jaka? (which? what is it like?) You can see some cooking blogs on the net calling their sandwiches with big amount of meat and nothing else "mięsna kanapka" - but it is not standard Polish , but calque from English
Depending on what it means. If it means "of", "out of", "from" - it takes Genitive. If it means "with" (and this here means "a sandwich with meat", it's just a less natural English translation I believe), it takes Instrumental.
"Kanapka z mięsa" would be "a sandwich made out of meat", a rather weird idea.