This idiom is understood by virtually all native speakers, but ordinary people rarely use it. They usually see this phrase used by villains in fictional works.
鴨 is a duck, and in Japanese it's also a metaphor for a person who is easily tricked, just like "gull" in English. There is a phrase ～を鴨にする (or ～をカモる for short), which means "to gull (someone)". 葱 is ...
Basically it means "easy hunt/game/prey". I think "something surprising but convenient" is slightly wrong. So it can't be used like your example. Second example is correct, the phrase exists for.
We love 蕎麦(Japanese noodle), and duck(鴨) meat one is really popular since Edo period. We usually put 葱(Green leek?) in 蕎麦, so if we found a 鴨 carrying 葱 and could ...
山を越える has an idiomatic meaning, which means "to pass the peak situation of something". For example, 彼女の病気の山は越えたよ(The worst situation of her illness was over), 明日でこの仕事は山は(orを)越えるだろう(The most important part of this job will be done tomorrow) and so on.
So this is an interesting translation, because what is actually being said, and the translation do not have the same literal meaning, but they carry the same general meaning as a figure of speech.
In short, a Japanese idiom is being translated to an English idiom.
Literally translates to:
I crossed over the mountain.
山 = mountain
は = ...