What were people eating in the classic era of the Mythos? The answer can be found in the New York Public Library's Buttolph Collection of historic menus. It's an amazing peek into everyday fare and fine dining that includes dinner menus dating back to the 1850s.
For our purposes the 1920s are the decade of interest, and the collection doesn't disappoint. You'll find a nice selection of period menus from restaurants, hotels, cruise ships, and train dining cars. This one hails from the Pennsylvania Railroad back in 1926:
From a prop perspective these are a goldmine. The high resolution scans can be used as-is or retouched for different locations more appropriate for player circumstances. They're just the kind of thing someone would jot an important note or clue on.
Beyond the theatrical utility they're interesting documents in and of themselves. One thing that struck me while browsing through the 20s is how popular bouillon was as a beverage. I'd never seen it offered at a dining establishment here in the US until the "bone broth" craze started a few years ago. Even odder is how prevalent clam bouillon is. It pops up again and again as the sole bouillon choice for travelers. I would have thought beef would be the flavor of choice.