To make a frittata in an air fryer, you'll need: 1 pound frozen spinach, 2 tablespoons butter, 1 teaspoon each thyme and kosher salt, and 1 tablespoon olive oil. Toss the spinach and mushrooms with the butter and oil and season with salt and pepper. Place the seasoned spinach mixture in the bottom of an 8x8-inch baking dish. Pour the mushroom mixture into the skillet and cook over medium-high heat until the mushrooms are browned, about 5 minutes. Remove from the heat and stir in 1/4 cup shredded Parmesan cheese. Serve immediately.