In a large pot add onions, carrots, garlic, and celery and either butter or vegetable oil. Saute until the onions are transparent. Add sausage, and cook until browned. If the sausage is fairly lean drain the excess oil.
Add chicken broth, Italian seasoning, diced tomatoes and brown lentils. Cover with lid. And simmer for about 1 hour. You may want to add some additional water if too much water cooks out during the cooking process.