In large stockpot, heat oil over medium heat. Add garlic and onion and cook until golden, stirring frequently, 10-15 minutes.
Add celery, bay leaves, basil, lemon juice, Worcestershire, and chicken broth. Heat to boiling, reduce heat, cover, and simmer 45 minutes, stirring occasionally.
Add bell pepper, mushrooms, and chicken. Cover and simmer 30 minutes.
In small saucepan, melt butter over medium-low heat. Add flour and cook, stirring constantly, 3-4 minutes.
Add about ¾ cup of the hot soup mixture, whisking constantly to prevent lumps.
Return to soup mixture and stir until well blended.
Stir in half and half and season with salt and pepper to taste.
Heat thoroughly, but do not boil.