1. Add tomatoes, celery, carrots, broth, onions, and basil to a large slow cooker. Cover and cook on LOW for 5-7 hours, until flavors are blended and vegetables are soft. Pour contents into food processor or blender, taking care as will be very hot. Puree veggies until smooth. About 30 minutes before serving prepare a roux. Melt butter over low heat in a skillet and add flour. Stir constantly with a whisk for 5-7 minutes. Slowly stir in 1 cup hot soup. Add another 3 cups and stir until smooth. Add all back into the slow cooker. Stir and add the cheese, warmed milk, and salt. Add additional basil if needed. Cover and cook on LOW for another 30 minutes or so until ready to serve.