1 oz sweet vermouth
1 oz Campari (I used Gran Classico Bitter, an excellent Swiss alternative)
2 oz prosecco
1 orange slice (or lemon twist)
Somewhere I have this recipe written down on a bar napkin from a trip to Genoa. I thought it might be a regional recipe or a bartender's secret recipe. Turns out it's fairly common, at least in Italy.
The Negroni calls for equal parts gin, Campari, and sweet vermouth and some recipes for the Sbagliato use equal parts as well but I prefer more prosecco to lighten it up.
Enjoy before your meal to stimulate the appetite.