This super refreshing, summery drink combines the flavor of elderflower with the bitterness of Campari, then force-carbonates it in an iSi siphon for an extra powerful fizz.
Add all ingredients to an iSi soda siphon (be sure that the thin interior tube is removed) and charge.
Shake the canister, then place it in the refrigerator for at least two hours, or preferably, overnight.
With the canister upright, very slowly release the pressure by pressing down the lever. (Do this a little bit at a time over the course of half an hour or so, storing in the fridge in between.)
Carefully open the top of the cannister and pour the drink into a highball glass over cubed ice.
Garnish with a mint sprig and an orange half-wheel.