A few minutes under the broiler turns the dish golden and crispy.
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.
Add milk; cook, stirring and scraping up any brown bits at the bottom of the pan.
Remove from the heat.
When the potatoes are done, remove them from the oven.
Transfer half the potatoes to a 2-quart broiler-safe baking dish.
Spread half the sauce over the potatoes and top with 1/2 cup cheese.
Add the remaining potatoes and top with the remaining sauce and cheese.
Let stand 10 minutes.
Serve topped with thyme.
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.
