At the house/museum of Gustave Moreau, a 19th century symbolist painter. He was very prolific and enjoyed painting large pieces featuring ancient gods and heroes and mythological creatures. I also browsed more than 1,000 sketches in recessed cabinets.
The place is just ten minutes' walk from Gare Saint Lazare in Paris, so a convenient and interesting gallery to visit during an afternoon in the French capital!
Ian