Complex sentence: Contrary to popular belief the large window of the Royal Lounge-listed Monument Historique-is not the work of Eiffel that never worked at the Negresco, but the entire work of Edouard-Jean Niermans.
Split into simpler sentences: Contrary to popular belief, the large window of the Royal Lounge-listed as an Historical Monument-is not the work of Gustave Eiffel. Eiffel never worked at the Negresco, instead it is entirely the work of Edouard-Jean Niermans.