펭귄이 그려진 스웨터를 입고 난 뒤부터 펭귄이 되었죠. (the simplest sentence)
= I have become a penguin since I wore a sweater with a penguin drawn on it.
펭귄이 된 건 펭귄이 그려진 스웨터를 입고 난 뒤부터죠. (rearranged to emphasize the time)
= It was after I wore a sweater with a penguin drawn on it that I have become a penguin.
-ㄴ 것은/건 ... 이었다 = It was ... that ... (것 = that)