Mix cognac and raspberry shrub in a highball glass with ice, top with club soda.
Club soda is to top - 3 oz is approximate. To make the raspberry shrub (recipe at link): take 12 oz raspberries and soak them in 24 oz of cider vinegar for eight days. Strain into a saucepan, pushing on the fruit to get all the liquid out. Add 32 oz sugar to the liquid, bring to a boil, stir to dissolve the sugar, then filter and refrigerate.
The drink likely dates from the 1870s in New Orleans - the name is variously attributed to the 1820s mayor of New Orleans, or the Roffignac cognac brand (or other stories.) The drink was saved from oblivion by Stanley Arthur's 1937 "Famous New Orleans Drinks and How to Mix 'Em" - which called for whiskey rather than cognac, but noted the original contained cognac. The raspberry shrub was a mystery ingredient for decades ( "Red Hembarig") before being unraveled by Robert Moss in 2011.
See also https://www.mikeaugustyniak.com/recipes/2017/5/roffignac for another set of proportions / raspberry shrub recipe.