oz. (340 ml) Campari
1 bottle (750 ml) chilled prosecco
6 oz. (170 ml) chilled club soda
6 fresh orange slices
Fill 6 wine glasses with ice. Add 2 oz (56 ml) Campari to each glass, then top evenly with the prosecco and soda (125 ml and 28 ml, respectively). Cut slits in orange slices and slide onto rims of glasses.
Note: I like to think of this cocktail as the Italian cousin of the Aperol Spritzer, and of course, Aperol may be substituted for Campari.