Spray 3 1/2- to 4-quart slow cooker with cooking spray. Spoon 1 cup of the Alfredo sauce in bottom of slow cooker. Spread 1 package of the pasta over sauce. Top with 1/2 cup of the shredded cheese, 2 cups of the tomatoes and 1/2 cup of the walnuts. Sprinkle with pepper. Repeat layers once. Top with 1 cup Alfredo sauce. Refrigerate remaining Alfredo sauce for another use.
Cover; cook on High heat setting 2 hours. Stir in spinach just before serving. Garnish individual servings with shaved Parmesan cheese.