In a large, non-reactive pot, combine the rhubarb, sugar, rosemary and lemon juice.
Stir so that the sugar coats everything evenly and let it sit for 30-45 minutes, until it looks juicy. Place pot on the heat bring to a bubble. Cook until the rhubarb has broken down (7-10 minutes).
Let the jam boil for 2 minutes and remove the pot from heat (with a batch this size, it is very easy to overcook the jam).
Add the lemon zest and stir to combine.
Remove the rosemary fronds.
Pour jam into prepared jars.
Wipe rims, apply lids, screw on bands and process in a boiling water canner for 10 minutes.
When time is up, remove jars to a towel-lined countertop and let cool.