Peel and dice 1 large apple into small pieces. In a bowl, place diced apple, 1/2 teaspoon ground cinnamon, 1 tablespoon brown sugar and 1/2 tablespoon lemon juice and mix together. Place mixture in an oven-safe dish and cover with foil. Bake covered in air fryer at 350˚F (175˚C) for 15 minutes, then remove foil and bake for another 5 minutes.
Meanwhile, in another bowl, stir together 1 cup rolled oats, 1/4 cup walnuts, 1/2 teaspoon ground cinnamon, 1/4 teaspoon allspice, 1/2 teaspoon baking powder and 1/4 teaspoon salt.. Next, stir in 3/4 cup milk, 1 large egg, 3 tablespoons maple syrup and 1 teaspoon vanilla extract and mix until ingredients are well combined.
Once apple has cooked, pour into bowl with the rest of the ingredients. Divide mixture evenly into 3 ramekins.
Bake each ramekin in air fryer at 320˚F (160˚C) for 10 minutes until top is golden. Allow 5 minutes to cool before serving.