Heat oil in a large pot over medium heat. Add mushrooms, green pepper, onion and garlic, and saute until tender, about 5 minutes. Pour in the chicken broth, and stir in the tomato paste. Bring to a boil, and add the lentils. Reduce heat to low, cover, and simmer for about 15 minutes. Season with basil, salt and pepper. Cover and simmer for another 15 minutes.
Ladle into bowls, and top each bowl with 1 tablespoon of sherry and sprinkle with Parmesan cheese to taste.
