Place the vinegar, tarragon leaves, scallions and parsley in a small pan over medium heat. Cook for 5 minutes or until all of the liquid has cooked out of the pan leaving only the tarragon leaves, parsley and scallion still moist (be careful not to burn).
Set aside while you make the Hollandaise sauce (recipe here).

When your Hollandaise is finished, whisk in the tarragon reduction.

Note: According to Galatoire's Cookbook this buttery sauce hails from the Béarn region of France, founder Jean Galatoire's homeland. Often used with Soufflé potatoes, Creole Fried Eggplant as well as on steaks, lamb chops and seafood.