Prepare equipment. Wash all canning supplies, jars, lids, and rings in hot soapy water. Sterilize if needed.
Prepare the jars and lids. Fill the canner about ⅔ of the way with water and bring to a boil. Add 7-8 pint jars (or 4 quart jars) filled with hot water and bring to a boil, holding the boil for at least 5 minutes. Turn off the heat and leave in the canner until needed. Add the jar lids to a pot of gently simmering water and keep warm over low heat during the canning process.
Prepare the apples. Peel, core, and slice the apples. Place them in a bowl of cold water (or treat with lemon juice/citric acid), then drain and measure to ensure you have a full 12 cups. Blanch*** if desired.
Once the jars have been sterilized, add the sugar, ClearJel, cinnamon, and nutmeg to a very large stainless steel saucepan. Whisk well, then pour in the apple juice and water. Whisk to combine.
Bring to a boil over medium-high heat, making sure to stir constantly (it’s very important that you watch this carefully and whisk/stir throughout the process).
Once the mixture begins to bubble and thicken considerably, add the lemon juice. Whisk until combined, then return to a boil for 1 minute (stirring or whisking constantly). Then immediately remove it from the heat.
Add the apple slices (drain if they are unblanched and holding in water), and fold them into the hot pie filling mixture.
Return the saucepan to the element and heat over medium heat, stirring often, until the apples are heated through.
Fit the jar with a canning funnel, and ladle the pie filling into the jar, leaving a generous 1” headspace (up to 1 ½ inches if using a regular mouth jar***). Remove any air bubbles with a clean butter knife or debubbler tool. Adjust headspace if necessary.
Wipe the rim of the jar with a clean, damp kitchen towel (or paper towels). Using a magnetic lid lifter, center a warm lid on the jar, then secure a canning ring until it is fingertip tight.
Place the filled jar in the canner, then repeat with the remaining jars until all of the jars are filled (or the pie filling has been used up). You should get 7-8 pints (or 3-4 quarts).
Place the lid on the canner and bring it back up to a rolling boil over high heat.
Once a boil has been reached, process for 25 minutes.
When the 25 minutes are up, turn off the heat and remove the canner from the element. Remove the lid, and wait 5 minutes.
After the 5 minutes are up, remove the jars with a jar lifter and place them on a thick kitchen towel. Leave them completely undisturbed for 24 hours.
After 24 hours, check the seals and remove the canning rings. Wipe down the jars and label them. Any jars that have not been sealed should be moved to the fridge to be eaten right away.