Make syrup:

• 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.