Freeze the beer mugs so that they are chilled at the moment of serving, ideally with a thin layer of ice.
First make the ginger syrup, putting all the ingredients in a mortar to obtain a soft paste. Set aside.
Then mix the grapefruit juice and ginger syrup in a jug.
Distribute the blueberries in 2 glasses, then the grapefruit-ginger mixture, and finally add the beer little by little. It is recommended to tilt the glass to prevent excess foam.