I'm putting together an asian sauce to throw over some steamed greens & have made it a bit too salty (it contains soy, fish & oyster sauce).
What can I add to cut back the saltiness?
The classic way to compensate for saltiness (especially in Asian cuisine) is to add something sweet (usually sugar), which tricks one's taste-buds into thinking that the food is both less salty and less sweet. (Ever wonder why a can of cola has 45+ mg of sodium? It's there partially to mask all the sweetness which gives you a sugar rush, and simultaneously make you thirstier!) I seem to recall reading a section on this phenomenon in On Food and Cooking, but my copy is back home.
Update #1: I haven't gotten a chance to look at OFaC yet, however, I did find this study:
It is a survey of research on both perceived and chemical reactions between different tastes.
The survey notes (see Sections 3.2 and 3.3):
It goes on to note that at even higher concentrations sweetness and other tastes are symmetrically suppressive (i.e., their tastes cancel each other out).
If you have (or add) spinach in with those greens I wouldn't worry. Spinach dishes can take a lot of salt before they taste over salted.
Many foods can absorb quite allot of salt and still be palatable. Many vegetables such as potatoes and rice are cooked in brine and people often add table salt to their potatoes on top of this. Other foods that we accept salty include sea food such as shrimp, fish, shellfish etc.
I live in China, and notice that people rarely add salt to foods. The condiments they use such as soy sauce and oyster sauce have sufficient salt in them already. So it is quite normal that your Asian style condiment mix will taste salty but it might not taste so salty once combined with the final dish.