Wash and dry the rosellas by soaking them in a sink full of cold water for a few minutes, then drain.
Separate the red calyxes (the fleshy cover) from the seedpods (green inside). An easy way to do this is to push an apple corer hard against the base of the calyx which will then separate the seedpod and the calyx.
Place the seedpods in a saucepan and the calyxes in a bowl for use later.
Wash both the seedpods and the calyxes separately.
Cover the seedpods with water until the seeds are just covered.
Bring to the boil and cook, covered for 30 minutes. They will become soft and transparent.
Strain the seedpods, reserving the liquid. The seedpods can be discarded.
Add the liquid back into the saucepan with the red calyxes. The calyxes may not be fully covered with the liquid, but that's okay.
Boil for 20 minutes until a 'pulp' consistecy is formed.
Take the saucepan off the heat and measure the volume of the cooked pulp using the cup measurement. You will need to know this for later.
Return the pulp mixture to the saucepan with the teaspoon of butter and the juice of one lemon.
You will need 1 cup of sugar for each cup of pulp. For example, if you had 2 cups of pulp, you will need 2 cups of sugar. Warm the sugar in a heatproof tray in the oven on low heat for a couple of minutes before adding to the pulp mixture.
Add the warmed sugar and stir until it is completely dissolved.
Quickly boil the mixture, uncovered for about 20 minutes or until the jam falls thickly off a spoon.
Another way to test the jam is ready is to place a saucer in the freezer to chill. Then place a small amount of the jam on the saucer, wait for it to cool slightly, then press the top of it with your finger. If the jam crinkles, it is cooked. Remember, jams set as they cool.
Once ready, bottle the jam into the prepared hot jars and seal.