1 tablespoon extra-virgin olive oil
1 1/2 tablespoons all-purpose flour
2 cups low-fat milk
1 teaspoon salt
1/4 teaspoon freshly ground pepper
1/4 teaspoon freshly grated nutmeg
2 pounds russet potatoes, peeled
2 cloves garlic, peeled and slivered
1 box frozen spinach, thawed and drained thoroughly
1/4 cup shredded Gruyère cheese
Preheat oven to 400°F. Coat a 1 1/2-quart gratin dish or other shallow baking dish with cooking spray.
Heat oil in a heavy saucepan, preferably nonstick, over medium heat. Add flour and cook, stirring constantly, until the flour begins to brown, 2 to 3 minutes. Gradually pour in milk and increase heat to medium-high. Cook, stirring constantly, until the mixture just begins to boil, about 5 minutes. (The sauce may seem thin.) Remove from the heat and season with salt, pepper and nutmeg. Place a piece of wax paper directly over the surface to prevent a skin from forming and set aside.
Cut potatoes into 1/4-inch-thick slices (a food processor does the job nicely); toss with garlic and spread evenly in the prepared dish. Pour the sauce over the potatoes and bake, uncovered, until the potatoes are very tender, 40 to 45 minutes. Sprinkle cheese over the top and bake until the top is golden, 10 to 12 minutes longer.