Crack your 4 large eggs and then place them into a medium-sized mixing bowl.
Add 1 cup mushrooms, 1 cup spinach, 1/4 cup leek, and mix well.
Fold in the 1/2 cup cheddar cheese, 3 tablespoons heavy cream, 1 teaspoon garlic salt, 1/2 teaspoon black pepper, and 1 teaspoon kosher salt Mix everything well.
Spray your baking pan with olive oil or non-stick cooking spray.
Pour the egg mixture into the prepared dish.
Place the pan into the air fryer, and set the temperature to 300 degrees F, air fryer setting, for 10 minutes.
Oven Instructions:To bake in the oven, place the pan in a preheated 350°F oven and bake for 20–25 minutes, or until the frittata is firm in the center and lightly golden on top.