Combine lentils, tomatoes, broth, onion, carrot, celery, garlic, oil, oregano, thyme, salt and pepper in a 6-quart slow cooker.
Cover and cook on low for 7 to 8 hours until lentils are tender, stirring once after 6 hours and checking a few grains before proceeding.
Stir in walnuts and balsamic vinegar and keep covered on warm while the pasta cooks.
Cook spaghetti in a large pot of boiling water according to the package until just tender, about 9 to 11 minutes. Reserve 1/2 cup cooking water and drain.
Toss spaghetti with lentil sauce, adding a tablespoon of reserved water at a time only if needed to loosen it.
Divide among 6 bowls, distributing the lentils and walnuts evenly.