The ancient Romans’ favorite condiment was called garum. Earlier, the Greeks had used it, and later, the Byzantines, but garum was most popular during ancient Roman times.
They dumped garum onto everything—the way Americans do ketchup, the Chinese soy sauce, and Egyptians tahini. Should you be curious to try garum yourself, I’ve written out the recipe for you. You’re welcome.
First, collect the heads, tails, intestines and other guts of whatever fish you have on hand, and salt them heavily. You can use anchovies, mackerel, sardines, or combinations of fish.
Layer the salted fish guts in a large amphora and then leave it out in the sun until the fish rot, ferment, putrefy, and liquefy. This process might take a few months. Stir occasionally.
Pour off the liquid that forms at the top—that’s the garum.
Garum is actually quite nutritious—full of amino acids, proteins, and vitamin D from all that time in the sun. But the rotten sludge left at the bottom is also highly nutritious, so save for another use, as recipes so often (and rather unhelpfully) suggest.