Cut your rhubarb stalks into smaller pieces/slices.
Place the rhubarb pieces into a large jam pot.
Add sugar and the juice of the lemon. You can add the lemon halves too if you like (extra natural pectin).
Mix everything well. At this point, you can choose to leave the ingredients to infuse for a while (Which might result in a better jam) or you can start to cook the jam.
Bring your jam to a rolling boil and stir occasionally.
Take down the heat and allow your jam to simmer or cook slowly until it is set
Check if your jam is set with a candy thermometer, 220 Fahrenheit/ 105 Celsius. Also, do the setting test by dropping some of the hot jam on an ice cold plate (which you kept in the freezer previously) and move around the plate to see if the jam is running. The jam is set if it's not moving, if not, cook your jam further and repeat the test until set.
Pour jam into clean sterilized jars and fill up to the rim. Close with a lid tight and turn upside down quickly. Leave the jam like that for a few hours, label and store in a cool and dry place until further use. Store in the fridge once opened.
Use garden fresh organic untreated rhubarb if possible
Do not reduce the sugar in the recipe. Read up in my post why.
Use the lemon juice and lemon rind in your jam. The rind contains plenty of natural pectins, which helps to preserve the jam.
Only use organic untreated lemon if you use the rind in the jam because of the pesticides.
The cooking time is relative and can be anywhere somewhere between 30 and 90 minutes. Please read up post for details about setting tempreture and time.