Low alcohol cocktails have been getting more attention over the last couple of years. Not just in the press but around the house as well. Usually this is at the end of the night when I feel like a drink would be nice but I don't necessarily want the full weight and proof of a regular strength cocktail. There's a variety of ways this can be accomplished. Often there's some sort of aperitif wine (vermouth, Byrrh, Bonal...) and/or an amaro involved. It can be as simple as flipping your vermouth to rye/bourbon ratio in a Manhattan for a Reverse Manhattan or just pouring some Averna over a chunk of ice and adding a twist. If it's the afternoon long drinks might be in order. The Bonsoni includes both sweet vermouth and an amaro and happens to be delicious any time of day. It's an incredibly easy to mix cocktail - 2 parts sweet vermouth to 1 part Fernet Branca. It shows up in Hugo Enslinn's Recipes for Mixed Drinks in 1916 and Patrick Duffy's Official Mixer's Manual a couple decades later in 1934. It also goes by the name 'Kill or Cure' which as Sean pointed out is an apt title. At the end of the night one will certainly have a somewhat soothing and medicinal effect but should that give way to multiple rounds the result is unlikely to be restorative.
This drink was also listed in Rogue Cocktails (which was changed to Beta Cocktails when the book was updated) with the suggestion to split the sweet vermouth with Punt e Mes. That does make for a richer more complex cocktail but if you're using a nice fresh vermouth, combining it 2:1 with Fernet is still going to make for a delicious drink. I usually make it with Cocchi's Vermouth di Torino because...well, I love that vermouth and reach for it more often than not these days. There's no mistaking the presence of Fernet in this drink. The bitter, medicinal, herbal and menthol qualities of that amaro remain intact. The vermouth however has contributions to make too. It sweetens things up and brings a nice grape flavor in addition to it's own botanical mix, allowing everything to stretch out.
Bonsoni - Hugo Enslinn, Recipes for Mixed Drinks, 1916
2 oz Sweet Vermouth
1 oz Fernet Branca
Stir, strain, up
*I usually build these over a large chunk of ice and add an orange twist.