Hull the strawberries and roughly chop. Remove both ends of the rhubarb and cut into slices, around ¾ in (2cm) thick.
Put all of the ingredients in a small-medium, wide pot/saucepan and place over a medium heat. Stir as it heats to dissolve the sugar, but try to avoid splashing the sugar up the side of the pot.
Bring the jam to a simmer, reduce the heat a little and simmer for approximately 15 minutes (or longer, as needed), stirring occasionally. Once the fruit softens, mash to break it up. Continue to cook a couple minutes so that the jam becomes a little darker in color and thicker - when you stir a spatula through, it will take a bit longer to close in behind.
Remove the pan from the heat, allow to cool a minute or two. Transfer to your prepared clean jar(s) and put lid on while still hot. Leave to cool.
Video
Notes
One quantity of this recipes makes approx 1 cup which is an 8oz (236g) jar. 4.4oz rhubarb is approximately 1 cup of rhubarb in chunks.