Wash and hull the strawberries. Cut each into 2 and put into a deep bowl. Slit the vanilla beans into 2 and add to the strawberries. Add the sugar to the strawberries and mix very well. Cover the bowl and refrigerate overnight.
Next day, take the strawberries out of the fridge. The sugar would have drawn the juice out of the strawberries.
Put this into a large and deep heavy walled pot. Add more sugar at this point, if your strawberries need it.
Add the salt, grated apple, the lemon juice and zest and mix. Turn on the heat and bring to a boil. The sugar will froth up and this is why a large pot is necessary. Allow to cook for about 15 minutes, stirring well in between.
Turn down the heat a bit and allow the strawberries to cook till very soft. Using a potato masher, break down the strawberries a bit so the jam is smoother but with small pieces of fruit.
Cook till the jam is done and passes the plate/ freezer test. Ladle the hot jam into hot sterilized jars and cover tightly.