Place a rack in the bottom of a large stock pot. Fill pot with hot water. Sterilize 7 1-quart canning jars, 7 lids, and 7 rings by placing on rack, jars upright. Bring water to a boil. Boil 10 minutes. Remove with a holder and allow jars to air-dry. Save water for processing apples.
Combine sugar, cornstarch, cinnamon, nutmeg, salt, and water in a large saucepan. Place over high heat and cook until thick and bubbly, stirring frequently. Remove from heat and stir in lemon juice.
Tightly pack apples into sterilized jars. Slowly pour syrup over apples, covering them completely. Gently tap jars on countertop to allow air bubbles to rise. Screw lids on jars.
Carefully lower jars into pot using a holder. Leave a 2 inch space between jars. Add more boiling water if necessary, until tops of jars are covered by 2 inches of water. Bring water to a full boil, then cover and process for 30 minutes.
Remove jars from pot and place on cloth-covered or wood surface, several inches apart, until cool. Once cool, press top of each lid with finger, ensuring that seal is tight (lid doesn't move up or down at all). Sealed jars can be stored for up to a year.
