Atlas Obscura has an article on Garum, the Sauce that ancient Romans put on all their food.
Slaves and laborers made the aromatic fish sauce by chopping up whole fish, including their guts, and tossing them into large clay pots with salt (amounts varied). The concoctions were then left to ferment for at least nine months under the hot Mediterranean sun while halophilic, or salt-loving, bacteria from the fish’s guts helped break down the flesh....
While some famous Romans, such as Seneca, were known to grumble about the “overpriced guts of rotten fish,” Pliny the Elder prized the “exquisite liquid.”the closest equivalent to Garum today is Asian fish sauce, which here we call Patis.
and here is a recipe on how to make it (although I suggest you just buy it: either in your local Asian grocery store, or at your local Walmart superstore grocery, where we used to buy it all the time.