Bring sugar and 3/4 cup water to a boil in a medium saucepan. Stir until sugar is dissolved. Remove from heat. Pour off all but 1 cup of simple syrup.

Heat syrup in a pan almost to a boil. Reduce heat to medium. Add cranberries and simmer until they just begin to burst. Let cool in the syrup.

Place 2 tbsp. drained cranberries and 6 tbsp. Cranberry syrup in a large pitcher. Add lemon wedges and orange slices. Using a muddler or wooden spoon, vigorously mash the fruit. Stir in gin and Sherry. Let steep for 5 minutes.

Strain into a medium pitcher. Fill 3 glasses with crushed ice. Pour half of the mixture into a cocktail shaker. Fill with ice. Shake vigorously for 10 seconds and strain into prepared glasses, then mound with more crushed ice. Garnish each with a mint sprig and three cranberries.