Ingredients
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2½ pounds Yukon Gold potatoes, cut into 1/4-inch slices (about 8 cups)
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3 tablespoons extra-virgin olive oil, divided
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1 small onion, finely chopped (about 1 cup)
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3 tablespoons all-purpose flour
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½ teaspoon salt
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¼ teaspoon white or black pepper
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2½ cups low-fat milk
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1 cup shredded sharp yellow Cheddar cheese, divided
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2 teaspoons fresh thyme
Directions
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Position racks in upper and lower third of oven; preheat to 425 degrees F.
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Toss potatoes with 1 tablespoon oil in a large bowl until well coated. Divide between 2 large baking sheets and spread in an even layer. Roast, rotating the pans from top to bottom about halfway through, until tender and beginning to brown, 20 to 25 minutes.
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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. Increase heat to medium-high; cook, stirring, until the sauce thickens and bubbles, 3 to 5 minutes. Remove from the heat.
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When the potatoes are done, remove them from the oven. Preheat the broiler.
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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. Broil, watching carefully, until the sauce is bubbling and the cheese is beginning to brown, 1 to 5 minutes, depending on your broiler. Let stand 10 minutes. Serve topped with thyme.
