Heat olive oil in large soup pot and saute onion, garlic, mushrooms, green pepper,salt and white pepper together until mushrooms are tender (about 5 min). Add stock and tomato paste. Mix until tomato paste is completely blend into stock. Stir in lentils and bring to a boil. Lower heat, cover and cook over medium-low heat for 15 minutes, stirring occassionally. Add basil and simmer, with pot covered, 15 minutes longer. Add extra salt and pepper to taste. Serve in individual bowls and float one tablespoon of dry sherry on top of each bowl. Garnish with remaining basil leaves.