Combine lemon verbena leaves (or scented geranium leaves) with sugar in a food processor.

Process until the mixture turns into a bright green paste, scraping down the sides as necessary.

Add lemon juice and process briefly.

Add cold water and process until combined.

Strain the liquid through a fine sieve to remove leaf particles.

Freeze in an ice cream maker according to manufacturer's directions.

If an ice cream maker is not available, strain and freeze the mixture, stirring every half an hour until done.