Add the plums, lemon juice, sugar and Rosemary brine to a saucepan
Cook for 20-30 minutes, until texture passes the cold plate test (see below)
Notes
To check if your jam has cooked long enough, use the cold plate trick: Put a plate in the freezer for 5 minutes, add a small amount of the jam onto the plate and tilt the plate. If the jam flows slowly down the plate it is done.