Cooking liquids are great for making sauces. Whether stock or the liquid from some poached meat (basically stock), these liquids are full of flavor and should be considered when a sauce is needed. Sauces, however, are (almost always) more viscous than the aforementioned liquids, so these liquids will need thickening. Stocks can be reduced so that the particles (particles of flavor!) floating within can be concentrated enough to make the sauce thick. Often, these stock reductions are emulsified (more on that later) with some kind of fat (usually butter), to give it more body and a richer feel. The liquid left over from a poaching, however, usually cannot be reduced.
Often, a generous amount of salt is added to the liquid to engender water- and flavor-absorption in the poaching meat. Were a poaching liquid of this composition to be reduced, it would be many times too salty. To tap into those flavors that will perfectly complement the dish you just poached, allow the liquid to cool, and then add a little starch to however much of it you plan on using. Whisk in the starch until all the lumps have broken up, and then place it back on the heat until it thickens, stirring constantly. For your starch, you can use flour, potato starch, corn starch, tapioca flour—any starch! If it is still too salty, add a little more water and heat until it thickens again (you may also need to add a little more starch, but be sure to let it cool, otherwise the lumps will cook, and will not separate save by vigorous blending).
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