What is a gastrique? How do you make it? And what is it good with?

Gastrique

a French term meaning to form a glaze by reduction. Some of the more common gastriques are the tarragon, pepper shallot and vinegar reductions for a classic bearnaise sauce or the red wine, herb and pepper reduction for a poivrade sauce. (1)

Gastrique is a reduction of vinegar and sugar brought to light caramelization, to which a little fond (stock) is added. It is a base to which many other ingredients, or just a few, can be added to form a sauce. It is generally used to create a sweet and sour sauce; one of the classics being orange sauce for duck (à l’orange)

An agrodolce (sweet and sour) is a similar sauce found in Italian cuisine. (2)

How to make a gastrique