In a shaker, combine gin, lime juice, and cranberry juice. Shake lightly to mix.
Pour mixture over ice in mug.
Top with ginger beer and stir gently with a bar spoon.
Garnish with cranberries and lime slice.
Notes
Use copper mugs for a colder drink. Adjust sweetness by changing cranberry or ginger beer amounts. Add extra lime for more tartness. Garnish with fresh or frozen cranberries for holiday flair.