Chicken and Root Vegetable Potpie

INGREDIENTS

• 3 cups fat-free, less-sodium chicken broth
• 1 1/2 cups frozen green peas, thawed
• 1 cup (1/2-inch) cubed peeled baking potato
• 1 cup (1/2-inch) cubed peeled sweet potato
• 1 cup (1/2-inch) cubed peeled celeriac (celery root)
• 1 cup (1/2-inch-thick) slices parsnip
• 1 (10-ounce) package frozen pearl onions
• 1 pound skinless, boneless chicken breasts, cut into bite-sized pieces
• 2/3 cup all-purpose flour (about 3 ounces), divided
• 1 1/2 cups fat-free milk
• 1/4 cup chopped fresh parsley
• 2 tablespoons chopped fresh thyme
• 1 1/2 teaspoons salt
• 1 teaspoon freshly ground black pepper
• Cooking spray
• 1 sheet frozen puff pastry dough, thawed