Scalloped Sweet Potatoes - Alpine Cardiology

Ingredients

• 2½ pounds sweet potatoes, peeled and cut into ¼-inch slices (about 8 cups)
• 3 tablespoons extra-virgin olive oil, divided
• 1 small onion, finely chopped (about 1 cup)
• 3 tablespoons all-purpose flour
• ½ teaspoon salt
• ¼ teaspoon white or black pepper
• 2½ cups low-fat milk
• 1 cup shredded Gruyère cheese, divided
• 2 teaspoons chopped fresh rosemary