Peel, core, and dice the apples into roughly ½-inch pieces.
Place the diced apples in an oven-safe bowl or cake pan. Add the water, lemon juice, cinnamon, brown sugar (if using), and salt. Stir to combine.
Cover the bowl tightly with aluminum foil.
Place in the air fryer basket and cook at 375°F (190°C) for 15–20 minutes, carefully stirring once halfway through. (Cook time depends on your apple variety — see notes.)
Check for doneness: apples should be fork-tender. If not, cook another 3–5 minutes.
Mash with a fork for chunky applesauce, or blend with an immersion blender for a smoother texture.
Taste and adjust seasoning. Let cool, then transfer to an airtight container.