The author describes this as "a piquant, almost spicy dressing that is equally good used hot over greens or at room temperature over mixed lettuces."
In a skillet, heat oil. Add garlic and scallion; saute until softened. Add broth and boil until the liquid is reduced by half - about 3 or 4 minutes. Stir in vinegar; cook an additional 2 minutes. Transfer to a bowl. Add ginger and tarragon. Gradually whisk in the olive oil. Add salt and pepper to taste.
