Place rhubarb, strawberries, lemon juice, fruit pectin and butter into a large saucepan over medium heat. (Butter is optional but helps keep jam from getting too foamy). Stir the fruit mixture to help the juice start to form and add sugar, about 200g at a time, stirring constantly until sugar is dissolved and the juice is starting to simmer. Turn up heat to medium-high, bring the mixture to a full rolling boil and cook and stir for 1 minute. Skim off any foam that forms.
Sterilise the jars and lids in boiling water for at least 5 minutes. With a large jam funnel and a soup ladle, pack the jam into the hot, sterilised jars, filling the jars to within 5mm of the top. Run a knife or a thin spatula around the insides of the jars after they have been filled to remove any air bubbles. Wipe the rims of the jars with a moist kitchen towel to remove any food residue. Top with lids and screw on rings.
Place a rack in the bottom of a large stock pot and fill halfway with water. Bring to the boil over high heat, then carefully lower the jars into the pot using a holder. Leave a 5cm space between the jars. Pour in more boiling water if necessary until the water level is at least 2.5cm above the tops of the jars. Bring the water to a full boil, cover the pot and process for 5 minutes.
Remove the jars from the stock pot and place onto a cloth-covered or wood surface, several centimetres apart, until cool. To help the jam set, don't move or touch the jars until cooled. Once cool, press the top of each lid with a finger, ensuring that the seal is tight (lid does not move up or down at all). Store in a cool, dark area.