Scald your jam jars and invert them onto a clean cloth.
In a saucepan, put the raspberries, the rhubarb cut into small pieces of 1 centimeter, add the sugar, the lemon and mix.
Cook over high heat for 20 minutes, at a strong boil, stirring regularly.
Fill your jam jars as full as possible to avoid leaving too much air, close them tightly and place them upside down. Wait for them to cool before putting them right side up and storing them in the refrigerator or cupboards...
Notes
It may be ridiculous to write this, but be careful not to burn yourself, it happens quickly!