All Amir knows about Polish people when he first comes to America is that "Polish men [are] tough steelworkers and that the women [cook] lots of cabbage." He has gotten this mental picture of the Poles through what he has heard, so his perception of them is limited, and amounts simply to stereotypes. Amir has never actually known a Polish person until he meets the old woman whose garden plot borders his.
Amir says that he speaks [quite often] with this woman, who walks seven blocks to the garden along the same route he takes. Both he and the woman plant carrots. Amir notices that when "her hundreds of seedlings" start to sprout, she refuses to thin them, to take out the weak ones to give the strong ones a better chance to grow. In a moment of trust and revelation, the Polish woman explains to Amir that she cannot bear to do the thinning, which she knows she should do, because it reminds her
"too closely of her concentration cmp, where the prisoners were inspected each morning and divided into two lines - the healthy to live and the others to die."
Amir recognizes that in sharing this information, the Polish woman is allowing him to see the rich culture and unique perspective from which she comes; she is revealing to him something about whom she is as an individual. From that moment, Amir sees her as a person, unique in her own right, and not just a Pole, and he realizes the limitedness of stereotypes - "how useless [is] all that [he's] heard about Poles." Once Amir gets to know the Polish woman for herself, he no longer "care[s]...whether she cooks cabbage." To him, the woman is more than her nationality; she is a living, breathing human being with a story and worth that transcends artificial stereotypes.
This is kinda like a short version because I'm not completely sure about this answer. But I do know that his stereotypes included that the Polish are always cooking.
I am sure that after he met a Polish, he took back everything he thought about Polish and decided they are like any other person.