Place the strawberries in a colander and rinse them under cold running water. Drain and hull the strawberries. Cut the larger strawberries in half or quarters and leave the small ones whole. Place the strawberries in a deep, heavy-bottomed pot such as Le Creuset and toss them with the sugar.
Bring the mixture to a boil over medium heat, stirring often. Add the apple (if the strawberries are under-ripe, add half of the apple; if they are very ripe, add the whole apple) and blueberries and continue to cook the mixture at a rolling boil, stirring occasionally, until the jam reaches 220 degrees on a candy thermometer. This should take 25 to 35 minutes. Skim and discard any foam that rises to the top. Test the consistency of the jam by putting some jam on a plate and placing it in the freezer for 5 to 10 minutes, until cool, to see if it jells. If the jam doesn’t quite jell, cook it’s a little longer and test it again. Allow the mixture to cool to room temperature and then store covered in the refrigerator. It will keep refrigerated for at least 2 weeks. To keep the jam longer, pack and seal in canning jars according to the manufacturer’s instructions.
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