In a large pot, sauté the onions, carrots, garlic, and celery in butter or olive oil until the onions are translucent. Season the vegetables with salt as they cook.

Add the sausage and cook until browned, breaking it apart as it cooks. Drain any excess oil from the pot.

Add the chicken broth, Italian seasoning, diced tomatoes, and lentils. Cover and simmer for about 1 hour. You may want to add some additional water if too much evaporates during the cooking process.