For the strawberry-raspberry jam, clean, wash and puree the strawberries. Wash the raspberries, strain them and put them in a pot together with the strawberries. Squeeze the lemon and add the juice and the preserving sugar to the pot as well.
Bring the jam to a boil, simmer for 10 minutes, stirring constantly. Fill the strawberry-raspberry jam into jars while still hot and seal.