In a large stockpot, whisk together sugar, cornstarch, cinnamon, and nutmeg. Slowly stir in water until smooth.
Bring the mixture to a boil over medium heat, stirring constantly. Once bubbling and thick, reduce the heat slightly and simmer for 2–3 minutes.
Stir in the lemon juice to brighten the flavor and help preserve the apples.
Carefully fold in the sliced apples. Stir gently to coat them thoroughly without breaking them apart.
Ladle the hot apple pie filling into sterilized quart-sized jars, leaving 1-inch headspace.
Wipe rims clean, place lids on, and secure bands until fingertip-tight. Process jars in a boiling water bath for 25 minutes.
Remove jars and let cool on a towel for 12–24 hours. Check seals, then label and store in a cool, dark place.
✅ Good for up to 1 year when properly sealed and stored.