French green lentils and black lentils hold up well to long, slow cooking without becoming mushy. Save the rinds from used-up blocks of Parmesan in a resealable plastic bag or tightly sealed container in the refrigerator. They give broth for soups a rich, savory flavor.
Combine celeriac, parsnips, carrot, pearl onions, celery, tomatoes, garlic and herbes de Provence in a 5- to 6-quart slow cooker. Add broth, lentils, rosemary, Parmesan rind, bay leaf, salt and pepper. Cover and cook on High for 4½ hours or on Low for 8 hours.
Remove the rosemary, Parmesan rind and bay leaf. Serve the soup topped with grated cheese and garnished with pancetta, if desired.