DIRECTIONS :
1 . Heat a large skillet over medium heat , then add the oil , celery , carrots and onions ; cook 5 to 6 minutes , or until golden . Add to slow cooker .
2 . Pour the juice of the tomatoes into the slow cooker , then roughly crush the tomatoes with your hands ; add to slow cooker . Then add chicken ( or vegetable broth ), the cheese rind ( if using ) thyme , basil , and bay leaf .
3 . Cover and cook on LOW for 6 hours , until the vegetables get soft and the flavors blend . Remove the cheese rid and , using an immersion blender , blend the soup until smooth ( or you can carefully do this in small batches in the blender ).
4 . Melt the butter over low heat in a large skillet and add the flour . Stir constantly with a whisk for 4 to 5 minutes . Slowly whisk in about 1 cup of the hot soup , then add the 1 3 / 4 cups of warmed milk and stir until smooth . Pour back into the slow cooker and stir , add the grated Pecorino cheese and adjust salt and pepper , to taste .
5 . Cover and cook on low 30 more minutes .
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