Heat water and sugar in a medium sauce pan over medium heat. Swirl occasionally until sugar is completely dissolved and just beginning to simmer.
Remove from heat and add fennel. Allow to steep for 30 minutes before straining. Store in fridge.
Shake mandarin juice, lemon juice, simple syrup, and celery bitters with ice. Strain into a rocks glass filled with ice. Top with ginger beer. Use a spoon to gently agitate and mix.
Garnish with a slightly crushed fennel sprig and a dehydrated lemon slice.