In a small saucepan over low heat, reduce the balsamic vinegar and agave nectar until syrupy, about 10 minutes. Set aside and let cool completely. Pour out onto a plate, creating a flat layer.
Dip the rim of a martini glass in the balsamic/agave reduction.
To a cocktail shaker filled with ice (preferably one large cube), add the vermouth, rye and Aperol. Stir thoroughly, then slowly strain into the rimmed martini glass.
The Boulevardier is a classic cocktail that is very similar to the Negroni, but made with rye whiskey instead of gin. The balsamic glaze adds an additional layer of complexity and balance by providing a sweet and sour component to a cocktail known for its dry and slightly bitter flavor profile.