For pudding: in a large bowl, add bread, apple, raisins, and walnuts and mix well.
In another bowl, add the remaining ingredients and mix until well combined.
Add the milk mixture into bread mixture and mix until well combined.
Refrigerate for about 15 minutes, tossing occasionally.
For topping: in a bowl, mix together the flour and sugar.
With a pastry cutter, cut in the butter until a crumbly mixture forms.
Lightly grease a baking dish with cooking spray.
Place the bread mixture into the greased baking dish evenly and spread the topping mixture on top.
Slide the Air Fryer Basket inside and adjust the temperature of Air Fryer to 355 °F to preheat for 5 minutes.
After preheating, arrange the baking dish into the Air Fryer Basket.
Slide the basket into Air Fryer and set the time for 22 minutes.
After cooking time is finished, remove the baking dish from Air Fryer and serve warm.