Why does 鱼 come before 做 in this sentence?
There is the "topic-comment structure" in Chinese:


  In a topic-comment sentence, the usual word order is rearranged a bit, in order to emphasize a certain part of a sentence. The part that is to be emphasized becomes the topic and moves to the beginning of the sentence, with the comment on the topic coming after the topic has been stated.


So here the topic is "这个鱼", which is the object of the verb. And the subject (who cooked the fish) is just omitted from the sentence.