Coarsely chop onion, slice carrots and celery on diagonal, large bites. Cube pancetta. Rough chop tomatoes, reserve liquid.
In heavy bottom soup pot, saute' onions in oil and butter until golden.
Add celery and carrots, cook 10 minutes.
Add tomatoes and juice, cook uncovered on low 25-30 minutes.
While veggies are simmering, crisp pancetta in a separate fry pan. (Pancetta takes longer than American bacon to crisp so I get it going with a little olive oil. You can use bacon or ham if you prefer.)
Add lentils, beef broth, pepper, salt if desired. Cook until lentils are soft, about 45 minutes. (Note: lentils absorb liquid at different rates. You may need to add more liquid or broth to get the consistency you like for soup. I use the red lentils for their color.)
Adjust salt and pepper to taste.
Serve with fresh grated parmesan and crusty Italian bread.