Preheat your campfire or grill to medium heat.
In a mixing bowl, combine the sliced apples, lemon juice, cinnamon, brown sugar, and salt. Toss until the apples are well coated.
Lay out a sheet of aluminum foil and place a quarter of the apple mixture in the center. Top with a tablespoon of butter.
If using, sprinkle granola or crushed graham crackers over the apples for added texture.
Fold the foil over the apples to create a sealed packet, ensuring there are no openings for steam to escape.
Repeat steps 3 to 5 for the remaining apple mixture and foil sheets.
Place the packets on the grill or campfire grate and cook for about 15-20 minutes, turning occasionally, until the apples are tender.
Carefully remove the packets from the heat. Let them cool for a minute before opening, as steam will escape.
Serve warm, optionally topped with a scoop of vanilla ice cream.