In a skillet, melt the butter over medium heat and cook the onion and bell pepper for about 4-5 minutes.
Remove the skillet from heat and set aside to cool slightly.
Meanwhile, in a bowl, add the eggs, milk, salt and black pepper and beat well.
Add the cooked onion mixture and gently stir to combine.
Place the zucchini mixture into a small baking pan.
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 pan into the Air Fryer Basket.
Slide the basket into Air Fryer and set the time for 6-8 minutes.
After cooking time is finished, remove the baking pan from Air Fryer.
Cut the omelet into 2 portions and serve hot.