Celery and Pear Bisque

INGREDIENTS

• 4 1/2 tablespoons butter
• 6 cups thinly sliced celery, with leaves plus chopped leaves (preferably organic, about 12 stalks, for garnish)
• 18 ounces unpeeled ripe bartlett pears, cored, diced (generous 3 cups)
• 1 1/2 cups leeks (the chopped dark green tops)
• 3 small turkish bay leaves
• 1 1/2 teaspoons chopped fresh thyme
• 1 1/2 tablespoons all-purpose flour
• 3 cups low-salt chicken broth (or more)