Don’t cut your sweet potatoes too thin–they’ll turn mushy under the sauce.
Divide between 2 large baking sheets and spread in an even layer.
Meanwhile, heat the remaining 2 tablespoons oil in a large saucepan over medium heat.
Add onion; cook, stirring frequently, until very soft and golden brown, 5 to 8 minutes.
Add flour, salt and pepper; cook, stirring, for 1 minute more.
Add milk; cook, stirring and scraping up any brown bits at the bottom of the pan.
When the sweet potatoes are done, remove them from the oven.
Transfer half the sweet potatoes to a 2-quart broiler-safe baking dish.
Spread half the sauce over the sweet potatoes and top with 1/2 cup cheese.
Add the remaining sweet potatoes and top with the remaining sauce and cheese.
Let stand 10 minutes.
Serve topped with rosemary.
Tips
To make ahead: Roast potatoes (Step 2) up to 30 minutes ahead.
- Daily Values (DVs) are the recommended amounts of nutrients to consume each day.
(-) Information is not currently available for this nutrient.
