I went to the DeYoung Museum today, and wandered through the Gauguin exhibit (on view until April 7, 2019). A comparative piece by Pissarro caught my eye:

Still Life with Flowers or Bouquet of Flowers ~1898:

https://fineartamerica.com/featured/bouquet-of-flowers-1898-camille-pissarro.html 

In addition to being an arresting painting, I also noted the painting in the background of the painting. [reference to the recent internet trend of painting people holding paintings] At first, I thought it might be a window, but first, the illumination is coming from a different direction, and second, you can clearly see a shadow fall across the painting on the left side.

So then I went looking to see if I could find the other painting. The first question is whether it is a real painting at all, and if so, whether one of Pissarro's, or someone else's. Of course, if it isn't real, then it’s Pissarro's anyway, in a way.

Compositionally, the painting appears to be of a road or park way, with a single figure on the left (park bench, wagon?), and perhaps a verge or river bank. In the middle there is a strangely shaped shadow, with a cluster of figures on the right, including someone in a reddish dress (?).

This certainly *appears* to be very similar to other Pissarro paintings, and on my first pass through his oeuvre (flipped through in the gallery on my phone), I found some promising similarities, like

https://www.camille-pissarro.org/View-Of-Pontoise-2.html

but no immediate direct hits.

(note riverbank, person with wagon, strange shadows, and reddish dress).

Several days later, after a bunch more searching (and a general unhappiness with the completeness of online collections of Pissarro’s oeuvre), I found it!

"The Quays at Rouen" 1883:

http://www.the-athenaeum.org/art/full.php?ID=12495

Note all the key bits: The horse and wagon next to the riverbank, the lone figure, the lamp-post, and most of all, the oddly-shaped shadow! And finally, it was comfortably painted in 1883… plenty of time to meld into the background.

Taking a second look on a laptop after getting back to the boat (another story), I found other "still lives with paintings", like https://www.camille-pissarro.org/Roses-Of-Nice.html, which clearly have other paintings in the background. Tally-ho!