To prepare a spinach lasagna with walnut pesto recipe, you'll first need to prepare the ingredients. First, preheat the oven to 400?F (204?C) and prepare your oven rack to the lowest setting. Using an electric mixer, beat the egg whites until stiff peaks form. Gradually add the milk and beat until the mixture is light and fluffy, about 2 minutes. Add the cheese and mix until it's completely melted. Pour the sauce over the pasta and stir to coat the noodles. Cover the dish with plastic wrap and refrigerate for at least 1 hour or overnight.