1. Spoon 1 cup Alfredo sauce into a lightly greased 3 1/2- to 4-qt. slow cooker. Top with 1 package pasta, 1/2 cup Parmesan cheese, 2 cups tomatoes, and 1/2 cup walnuts. Sprinkle with pepper. Repeat layers once. Top with 1 cup Alfredo sauce. Reserve remaining Alfredo sauce for other uses.
2. Cover and cook on High 2 hours. Stir in spinach just before serving.