The Boulevardier was an English language literary magazine in Paris in the 1920s. Erskine Gwynne, the editor, was a loyal customer of Harry McElhone, who founded the eponymous Harry's New York Bar. Even though it's not clear if Gwynne or McElhone created the Boulevardier, McElhone mentioned it in a footnote in his 1927 book Barflies and Cocktails.
Combine in a mixing glass with ice, stir with some American je ne sais quoi, and strain into a chilled glass. Lemon or orange peel garnish optional.
Fundamentally a Boulevardier is a Negroni with bourbon instead of gin. Another way of viewing it is that it's a modified Manhattan with Campari instead of Angostura bitters. Like other cocktails such as the Old Pal (which McElhone created for another one of his loyal customers), the Bijou, and the Last Word, the Boulevardier is a bartender’s dream because of its simple ratio and short ingredient list. If you want to emphasize the bourbon, a variation I like uses one and half ounces of bourbon and .75 ounces each of the sweet vermouth and Campari. The Boulevardier lends itself to tinkering. For example, add some molé bitters, and you have a Left Hand.
Looking for a simple and laudable cocktail? Have a Boulevardier and look no further.