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Diana B is a trusted home cook.
I just simmer balsamic until it's reduced by about half or has the syrupy consistency I want.
Two ways of attacking this. You can season the meat with, say, rosemary, thyme and garlic then make a plain reduction, or you can season the reduction. I prefer the former because I can then use the glaze for subsequent meals (over grilled pineapple, salads, etc.)
To make the reduction properly, you need to keep it at a sub-simmer (steaming, no bubbles) otherwise the delicate flavors will be affected. If your burner can't keep such a low flame, use a heat diffuser or pull the pan half-way off the burner while it reduces. If it's not obvious, it can take 2 or more hours to reduce a couple of cups of vinegar this way but it's worth the effort.
You can reduce the balsamic ba as much as 4:1. If you go too far, simply dilute with a little water. After it cools a bit, transfer to a plastic squeeze bottle if you have one. It will keep in the refrigerator (but warm it to at least RT before use).
Lamb is also great with orange so we use an orange balsamic from Whole Foods but you could also add some orange zest to your reduction.