Sweet Potato Gratin

Ingredients:

• 1 tablespoon butter, softened
• 1 cup thinly sliced sweet onion
• 2 pounds sweet potatoes (about 3 large), peeled and thinly sliced
• 1 tablespoon all-purpose flour
• 1 teaspoon chopped fresh thyme
• 1/2 teaspoon kosher salt
• 1/2 teaspoon freshly ground black pepper
• 2 ounces grated fresh Parmesan cheese (about 1/2 cup), divided
• Cooking spray
• 1/2 cup organic vegetable broth

Source