Puree raspberries, then pass through a fine strainer. Discard seeds.
Pour liquid raspberry into an ice-cube tray and place in the freezer. When fully frozen, put 2-3 frozen raspberry cubes, along with a handful of crushed ice, into a blender with remaining ingredients and blitz. Any remaining frozen puree can be put back in the freezer for next time.
Pour into a margarita glass and garnish with raspberries threaded on to a cocktail skewer, if desired.