Popeye's Cajun Flounder Sandwich

If they're calling it flounder, it's probably flounder. There are many species of flounder, some of which are abundant on the west coast.

For example (per NOAA Fisheries):

Arrowtooth flounder: Abundant from Alaska to West Coast. Not overfished. Good choice for sustainable seafood harvest. Harvest includes commercial trawl.

https://www.fisheries.noaa.gov/species/arrowtooth-flounder