In a large pot, combine sugar, cornstarch, cinnamon, nutmeg, and salt. Add water and stir until smooth. Bring to a boil over medium heat; cook and stir for 2 minutes or until thickened and bubbly.
Add lemon juice and apples; return to a boil. Reduce heat; cover and simmer until apples are tender, about 6-8 minutes.
Ladle hot filling into hot jars, leaving ½-in. headspace. Remove air bubbles; wipe rims and adjust lids. Process for 20 minutes in a boiling-water canner.