Prepare rosellas by washing and removing the calyx. Discard fruit that does not have green seed pods. Save the brown pods for seed for next year.
Keep about one quarter of the green seed pods. Place them in a saucepan, cover with water and boil until they are soft. This is where the setting agent for your jam comes from.
When they are soft, discard the pods and add the water to the rosellas in another saucepan.
Add enough additional water to just cover the rosellas, bring to the boil and simmer until the fruit is soft. This only takes about 10 minutes.
Puree with a stick blender. The mixture will have a large amount of froth on top.
Add sugar to taste. For me this was about 1 cup of sugar for 4 cups of rosella puree.
Return to the stove and simmer until all of the sugar is dissolved and the jam will set when dropped on to a cold saucer. This does not take more than about 10 minutes with the amount of setting agent added. If you want the jam to keep for a long time at room temperature you will need to add 40% to 50% sugar.
Skim off the froth into a bowl and freeze. This is the sorbet. For 4 cups of rosella puree you should get about 2 to 3 serves of sorbet. And it is totally delicious.