Creamy Parmesan Potato Bake with Spinach

Method

1. Preheat the oven to 425°F. Position the oven racks in the upper and lower thirds.
2. In a large bowl, toss the potato slices with 1 tablespoon of olive oil until well coated. Divide them between two large baking sheets in a single layer. Roast for 20–25 minutes, rotating the pans halfway through, until the potatoes are tender and lightly browned.
3. Meanwhile, heat the remaining 2 tablespoons of olive oil in a large saucepan over medium heat. Add the finely chopped onion and cook, stirring often, until very soft and golden brown, about 5–8 minutes.
4. Add the minced garlic and cook for 1 minute, stirring, until fragrant.
5. Stir in the flour, salt, and pepper. Cook, stirring constantly, for 1 minute to remove the raw flour taste.
6. Slowly pour in the milk, stirring and scraping up any browned bits from the bottom of the pan. Increase the heat to medium-high and cook, stirring, until the sauce thickens and starts to bubble, about 3–5 minutes. Remove from heat.
7. Squeeze the thawed spinach well to remove excess moisture, then stir it into the sauce.
8. Remove the cooked potatoes from the oven. Turn the oven to broil. Transfer half of the roasted potatoes to a 2-quart broiler-safe baking dish. Spread half of the spinach sauce over the potatoes, then sprinkle with ½ cup of Parmesan cheese.
9. Layer the remaining potatoes on top, then spread the remaining sauce and sprinkle with the remaining ½ cup of Parmesan cheese.
10. Broil the dish 4–6 inches from the heat, watching carefully, until the sauce is bubbling and the cheese is lightly browned, about 1–5 minutes. Let it rest for 10 minutes before serving.