When I first came across garum in ancient Roman recipes, I substituted fish sauce. Sometimes worcestershire, sometimes fish sauce from the Asian supermarket. Then I found an anchovy sauce in Naples which claimed to be the real thing. Magnifico!
And I can now order it from Salerno, via Amazon.
The ancient Romans made this salty, pungent sauce by fermenting fishgut, tails, heads, and other small whole fish in salt for several days out in the sun. Their fish sauce factories, salsamentarii, churned out massive amounts or anyone could make their own in the courtyard.
From Gargilius Martialis, De medicina et de virtute herbarum
Use fatty fish, for example, sardines, and a well-sealed (pitched) container with a 26-35 quart capacity.
Add dried, aromatic herbs possessing a strong flavor, such as dill, coriander, fennel, celery, mint, oregano, and others, making a layer on the bottom of the container; then put down a layer of fish (if small, leave them whole, if large, use pieces) and over this, add a layer of salt two fingers high.
Repeat these layers until the container is filled. Let it rest for seven days in the sun. Then mix the sauce daily for 20 days. After that, it becomes a liquid
One day I’ll make my own in the traditional way but, for the moment, I’m happy with the Colatura.