Preheat your air fryer to 350°F (180°C) for about 5 minutes. This ensures that it's hot and ready for cooking.
Clean the mushrooms by wiping them with a damp cloth or paper towel to remove any dirt. Remove the stems from the mushrooms, and set them aside for later use.
In a medium mixing bowl, add the spinach, cream cheese, Cheddar cheese, garlic powder, salt, pepper and walnut. Mix thoroughly.
Stuff each mushroom cap with the cream cheese mixture. You can use a spoon or your fingers to do this, making sure to fill them generously.
Lightly spray the air fryer basket with olive oil to prevent sticking.
Place the stuffed mushrooms in a single layer in the air fryer basket, leaving some space between each mushroom to ensure even cooking.
Air fry the stuffed mushrooms at 350°F (180°C) for 8-10 minutes, or until they are golden brown and the cheese is bubbly and slightly browned.
Check the mushrooms occasionally to avoid overcooking. Cooking times may vary depending on the size of your mushrooms and the specific air fryer model you're using.
Once done, carefully remove the stuffed mushrooms from the air fryer using tongs or a spatula. Let them cool for a few minutes before serving.
Garnish with additional chopped parsley if desired, and serve your air fryer stuffed mushrooms as an appetizer or snack.