In a large pitcher or drink dispenser, combine the spiced pear juice, gin, lemon juice, maple syrup, vanilla bean paste, pear liqueur (if using), and stir until well incorporated.
Add the cinnamon sticks and fresh rosemary sprigs directly to the pitcher, gently pressing the rosemary to release its aromatic oils.
Cover and refrigerate for at least 2 hours to allow the flavors to infuse and the cocktail to chill.
Before serving, taste and remove the rosemary if the herbal flavor is pronounced. The cinnamon sticks can remain in the pitcher for presentation.
Pour approximately 5 oz of the mixture into each ice-filled glass. Top with 2–3 oz of chilled Prosecco or sparkling water.
Garnish with fresh pear slices, cinnamon sticks, or rosemary sprigs.