Q: What, exactly, is oyster sauce? Is it really derived from oysters? Is fish sauce truly distilled fish?
A: In its purest form, oyster sauce is made by boiling oysters, removing them and cooking down the water in which they were boiled, which concentrates the oyster extracts.
Many versions of oyster sauce, including some made by Lee Kum Kee, my favorite brand, also contain starch, MSG, caramel color and synthetic preservatives. As a general rule, get the stuff with the fewest ingredients. Oyster sauce is usually added to steamed, blanched or stir-fried veggies and meat. I like to enhance my oyster sauce by frying minced garlic in oil and adding white pepper and the sauce. Pour this over your stir-fry and you'll be happy as an, uh, oyster.
Fish sauce is made, unsurprisingly, from fish, but the process is much more elaborate than for oyster sauce. Small fish, like anchovies, are typically used—larger fish can be used, too, but large fish usually have greater commercial value elsewhere. The fish are heavily salted and stored in earthenware crocks in the sun. The salt draws out the fish juice, and everything ferments in the warm crock. The fish sauce is drained from the crock after nine to 12 months. Shortcuts are used in the production of cheaper fish sauces, but if you want the good stuff, go for Golden Boy or Tra Chang.
Fish sauce is a staple in Southeast Asian cooking, where it's used in place of salt or soy sauce. It's an acquired taste, to put it mildly. It basically smells like rotten fish, and so will your kitchen—for a few minutes—when you add it to food. But like some stinky French cheeses, the flavor is much milder than the smell.