If you don’t care for the licorice taste of fennel, omit it and add another cucumber.
Juice cucumbers and fennel in an electric juicer. (You should have about 3 cups liquid.) Transfer to a large pitcher. Stir in club soda, honey, lemon juice, salt, and limoncello, if desired. Serve over ice. Garnish with lemon-peel strips and fennel fronds.