Peel, core and slice the apples. Transfer them to a bowl and toss them in freshly squeezed lemon juice to keep them from turning brown.
Melt butter in a large saucepan or pot and add the apple slices, brown sugar, ground cinnamon and nutmeg and 2-3 tablespoons of water.
Cook on medium heat until the apples just start to soften a bit. In a small cup, combine the cornstarch with a bit of water to form a slurry.
Pour the slurry into the saucepan and stir immediately. Cook for an additional minute, then take off heat.
Transfer to shallow containers and refrigerate promptly to cool; do not leave at room temperature for more than 2 hours (1 hour above 90°F). Once cold, transfer to a freezer-safe bag, remove excess air, seal and lay flat to freeze. Use within 3 months for best quality.
Notes
Use firm baking apples like Granny Smith, Honeycrisp, or Braeburn for the best results. A combination of sweet and tart apples gives the filling a nicely balanced flavor.
Peel, core, and slice the apples into evenly sized pieces so they cook uniformly and hold their shape.
Toss the sliced apples with lemon juice immediately after cutting to help prevent browning.
Cook the apples over medium heat, stirring gently to coat them in the butter, brown sugar, and spices without breaking the slices.
Avoid overcooking the apples, as they will soften further when baked into a pie, crisp, or crumble.
Mix the cornstarch with cold water before adding it to the hot apple mixture to prevent lumps from forming.
Adjust the brown sugar depending on the sweetness of your apples, or add a little extra cinnamon for a stronger spiced flavor.
Add a pinch of ground ginger, cloves, or allspice for a warmer, more aromatic filling.
Transfer the cooked filling to shallow containers and refrigerate promptly to cool before freezing.
Divide the cooled filling into freezer-safe bags, remove excess air, and flatten them for easy stacking and quicker thawing.
Label each bag with the date and quantity, and freeze for up to 3 months for the best quality.
Thaw the filling overnight in the refrigerator before using. Stir gently to redistribute any separated juices.
If the thawed filling seems watery, simmer it briefly to reduce the excess liquid, then cool before adding it to a pie crust.
Use the filling in homemade apple pies, turnovers, crisps, crumbles, or as a warm topping for pancakes, waffles, and ice cream.