(1) For the infusion, combine 3 slices fennel bulb, roughly chopped, in 1 cup elderflower liqueur. Let sit, covered, for 1 hour, then strain through a fine mesh sieve. Reserve infused fennel for muddling.
(2) In the bottom of a chilled collins glass, add a tablespoon of the infused fennel and 1 slice citrus. Muddle, then top with ice.
(3) Pour 1.5 oz infused elderflower liqueur over the ice, then top with seltzer. Stir lightly and serve with a straw.