once served as hot sausages on a plate, the hot dog became what it is today by simply
moving from the plate to its bread.

해설서에는,,,
= once the hot dog was served as~~ / though the hot dow was served as~ 라고 나와 있는데...
저는
the hot dog was once served as~~에서 동일 주어 생략, was가 being으로 바뀌어 생략 된걸로 생각했거든요...

once가 문장 맨 앞으로 가면 한때..라는 의미보다 접속사 처럼 느껴져서 was뒤에 붙이는게
문맥상 자연스러운거 같은데...
어떻게 생각하시나요?
제가 잘못 생각하고 있는건지....