Add the London dry gin, lime juice and a handful of ice to a copper mug. If you don't have a copper mug, a double old fashioned glass works.
Using a bar spoon, stir the mixture until it's cold and condensation begins to form on the copper mug, about 15-20 seconds.
Top with chilled ginger beer. Gently stir to combine. If desired, garnish with a lime slice or wheel. Enjoy!