¼ fennel bulb, untrimmed, coarsely chopped
1 375-ml bottle dry vermouth (preferably Dolin)
1 large rhubarb stalk, thinly sliced, plus more, shaved, for serving
½ cup sugar
¾ cup fresh lemon juice
¾ cup gin (preferably Fords)
Combine fennel and vermouth in a glass container (save vermouth bottle). Cover and chill 2 days. Strain through a fine-mesh sieve back into bottle.
Bring rhubarb, sugar, and ½ cup water to a gentle simmer in a small saucepan over medium heat and cook until mixture is pink and slightly syrupy, 14–16 minutes. Let cool, then transfer to a blender and purée until smooth (you should have about ½ cup rhubarb purée).
Combine 12 oz. fennel-infused vermouth, rhubarb purée, lemon juice, gin, and 2 cups ice in a large pitcher. Stir until cold. Pour cocktail into ice-filled old-fashioned glasses and garnish with shaved rhubarb.
Do Ahead: Vermouth can be infused and rhubarb purée can be made 1 week ahead; cover and chill separately.
Rhubarb-Fennel Gin Cocktail