Put the unthawed raspberries into a saucepan and add sugar.
Cook on medium, stirring occasionally until all the sugar ahs dissolved.
Using a potato masher, mash the raspberries and syrup thoroughly.
Pour the mixture through a strainer, extract all the juice.
Squeeze two of the lemons and strain the juice, add it to the raspberry juice.
Add the boiling water to the raspberry juice.
Allow the cordial to cool, then chill it in the refrigerator.
Serve cordial with a thin slice from the remaining lemon in each glass.