Prepare strawberries and bananas. Transfer the berries to a saucepan, add sugar, leave for 6 hours to let the strawberries release their juice. Transfer the bananas to the saucepan with the strawberries.
Cook the jam. Place the pan with the berries on low heat, bring to a boil, stir occasionally. Cook for 5 minutes, turn off the heat, let the jam cool completely for 5 hours. Add lemon juice to the jam and place the pan on low heat again. Bring to a boil, cook for 5 minutes.
Preserve the jam. Pour the finished jam into jars and close with lids. Turn the jars upside down, wrap them in a blanket and let them cool. After that, move the jam to the pantry.