Wash the lemon with hot water, dry it, and cut into slices.
Pat the fresh melon sage dry as much as possible. Roughly chop it and place in a large glass jar with a lid along with the rock sugar and lemon slices. Pour the vodka over and seal the jar.
Let the mixture steep for 8 weeks, ideally on a windowsill. Shake the jar once a week so that the rock sugar dissolves better.
After 8 weeks, filter the mixture and transfer the liquid to a clean bottle.
Let the liqueur age for at least 4 weeks before drinking. The longer it ages, the better the flavor becomes.