I am looking at making a variation of Alain Passard's egg, the Farm Egg with Corriander.  The instructions are somewhat clear, with the key exception being the phrase "reduce until au sec".  With a little googling, I can see that means "nearly dry", and in context, I would guess that means that there's very little of the liquid (water, lemon zest, vinegar) left.  So, what is the best way to reduce?  High or low temperature?  Whisk / stir?  Thanks for any advice! 
I would read that to mean reducing to the point of it being syrupy. You can go with moderate heat until you sense it thickening, then you probably should turn it down to be very careful not to burn it. It's the kind of thing that will seem a lot easier the second time you do it with the same recipe.  Use your smallest saucepan.   You probably won't need a whisk, but keep one handy just in case. A wooden spoon is probably your best implement.  If at any point you feel like it's getting away from you (getting lumpy or otherwise weird), remove it from the heat and whisk it a bit.  Usually recipes will say to reduce by half or two thirds, or it will say until you have a certain amount left. Since this one doesn't, go with your gut. You're done when it has thickened and will nicely flavor a pint of crème fraîche without making it watery.  That term is used in recipes for Beurre Blanc; look at this recipe from Epicurious. It's the same idea.  Let us know how it goes. It's an interesting question. 