Infuse the gin with elderberries and black tea: mix them together in the gin bottle. Gently warm the closed bottle in a pan of boiling water. Let the bottle cool down for two hours while the ingredients enrich the gin. Strain the gin through a sieve until you have a black but clear liquid. Pour 40 ml of the infused gin and 40 ml of Rabarcello in a large glass with ice. Top off with a generous splash of quality tonic. Garnish with a thin strand of fresh rhubarb and a slice of lime.