Wash and hull the strawberries, halve them. Clean and peel the rhubarb, cut it into small pieces.
Grate the zest of one lemon and juice both of them. Grate the apple as well.
Place a small plate in the freezer at the beginning of the cooking time.
Place all the ingredients in a large pan, bring to a boil while stirring often and let cook for about 50-60 minutes until the jam starts to thicken.
There will be quite a lot of foam forming during this time, remove it with a slotted spoon.
Start checking the jam after 30 minutes. Read the blog post for more information on how long to boil the jam.
Check the consistency of the jam by dropping a few drops of juice on the plate from the freezer. Put the plate back in the freezer and check again after 1 minute. The jam drops should wrinkle when pushed with the finger. If they don't wrinkle, keep cooking and checking until you get them to wrinkle.
While the jam is cooking sterilize the jars and the lids.
Pour the jam into the hot prepared jars and seal tightly.
If desired, can in a canner or in a hot water bath.