1. Preheat your air fryer to 350°F (175°C). Preheating ensures even cooking.

2. Prepare your baking dish. Spray it with cooking spray or brush with olive oil to prevent sticking.

3. In a medium mixing bowl, whisk together 6 large eggs and 1/4 cup of milk until well combined.

4. Add the spinach, 1/2 cup shredded mozzarella cheese, 1/4 cup chopped onion, 1/4 cup chopped bell pepper (if using), and 1 clove minced garlic to the egg mixture. Stir well.

5. Season with 1/2 teaspoon salt, 1/4 teaspoon black pepper, and 1/4 teaspoon red pepper flakes (if desired). Mix thoroughly.

6. Pour the mixture into your prepared baking dish. Ensure it’s spread evenly.

7. Place the baking dish into the preheated air fryer basket. Cook at 350°F for 15-18 minutes. Check at 15 minutes by inserting a toothpick into the center; it should come out clean if done.

8. Carefully remove the baking dish from the air fryer using oven mitts. Let it cool for a few minutes before slicing and serving.