In a medium-sized saucepan, heat the vegetable oil over a medium flame. Once heated, add the chopped onion and celery and sauté for approximately 5 minutes until they become softened. Then, add in the dried lentils and continue to cook for an additional 1-2 minutes.
Pour in the canned Italian plum tomatoes (including the juice) and beef broth. Also, add 3 cups of water and a pinch of salt to taste. Bring the mixture up to a boil then reduce the heat and simmer for 15-20 minutes.
Add butter, pepper, and grated Parmesan cheese to the soup. Simmer over low heat for about 5 minutes while stirring occasionally. This will allow all the flavors to blend together nicely.
Lastly, toss in the prosciutto or unsmoked ham and cook for about 2-3 minutes more. Taste the soup afterward and adjust any of the seasonings as needed before serving.
Serve your homemade Marcella Hazan Lentil Soup in individual bowls and top with a sprinkle of extra Parmesan cheese, if desired.