Select 6 medium firm apples without bruises or soft spots.
Peel and core each apple, then cut it into even slices about ½ inch thick.
Place the slices in a large bowl and gently toss with 1 tablespoon lemon juice.
Combine ¾ cup brown sugar, 2 teaspoons cinnamon, ¼ teaspoon nutmeg, and ¼ teaspoon salt in a small bowl.
Arrange the prepared apples evenly inside a 5–6 quart slow cooker.
Sprinkle the seasoned brown sugar over the apples. Distribute ¼ cup butter over the top and pour ½ cup apple juice around the edges.
Cover and cook on HIGH for 2–2½ hours. Begin checking at 2 hours. The apples should be tender but still hold their shape.
Whisk 2 tablespoons cornstarch with 2 tablespoons cold water until smooth. Gently stir it into the hot apple mixture. Cover and cook on HIGH for another 15–20 minutes.
Turn off the slow cooker and let the apples rest uncovered for 10 minutes before serving warm.