1. In a large pot, combine water, granulated sugar, brown sugar, cornstarch, cinnamon, nutmeg, and salt. Bring to a boil over medium heat, stirring constantly until the mixture thickens.
2. Add lemon juice and sliced apples to the pot. Stir to coat the apples with the syrup.
3. Reduce heat and let it simmer for about 10 minutes, until the apples are tender but not mushy.
4. Carefully ladle the hot apple pie filling into sterilized canning jars, leaving about ½ inch of headspace. Wipe the rims clean, place the lids on, and screw on the bands until fingertip tight.
5. Process the jars in a boiling water bath for 20 minutes. Remove the jars and let them cool completely. Check the seals before storing.