To prepare a spinach fennel and zucchini frittata recipe, you will first need to prepare the ingredients. You will need the following ingredients: 1 1/2 cups frozen spinach 1/2 cup chopped scallions 1/4 cup diced zucchini 1 tablespoon kosher salt 2 tablespoons olive oil 1 teaspoon thyme 2 teaspoons freshly ground black pepper Instructions: Preheat the oven to 425 degrees F. Line an 8-inch-square baking pan with foil and set aside. In the bowl of an electric mixer fitted with the paddle attachment, beat the egg whites on medium speed until stiff peaks form. Gradually add the milk, beating well after each addition. Pour the batter into the prepared pan and bake for 20 to 25 minutes or until the top is golden brown. Remove the pan from the heat and allow the mixture to cool for 5 minutes before serving.