Add the rhubarb and strawberries to a large pot. Without adding any extra water turn the heat on to medium-high and leave it to do its thing.
After five minutes or so liquid will start to come from the fruit. Sprinkle over the sugar (more or less, according to taste - this makes for a sweet compote) and stir well.
Leave to simmer until the rhubarb and strawberries have softened and blended together.
Remove from the heat and stir in the lemon juice.
Spoon into hot, sterilized jars, pop the lids on and leave to cool before labeling.