Position racks in upper and lower third of oven; preheat to 425 degrees F.Advertisement
Toss potatoes in a large bowl with 1 tablespoon oil until well coated. Divide between 2 baking sheets and spread in an even layer. Roast, stirring once and rotating the pans top to bottom about halfway through, until tender and beginning to brown, 20 to 25 minutes.
Meanwhile, heat 1 tablespoon oil in a large saucepan over medium heat. Add onion and 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 and continue to stir, scraping up any browned bits. Cook, stirring, until the sauce bubbles and thickens enough to coat the back of a spoon, about 4 minutes. Remove from the heat. (See Tips)
When the potatoes are done, remove from the oven. Preheat the broiler.
Transfer half the potatoes to a 2-quart, broiler-safe baking dish. Spread half the sauce over the potatoes. Add the remaining potatoes and top with the remaining sauce.
Combine breadcrumbs and the remaining 1 tablespoon oil in a small bowl (skip this step if you are topping with cheese).
Sprinkle the breadcrumb mixture (or cheese) over the gratin. Place under the broiler and broil, watching closely, until the gratin is bubbling and beginning to brown on top, 1 to 5 minutes, depending on your broiler. Let stand for 10 minutes before serving.
Make Ahead Tip: Roast potatoes (Step 2) up to 30 minutes ahead. Prepare the sauce (Step 3), cover and refrigerate for up to 1 day; gently reheat until steaming before combining with the potatoes.
Tips: To make your own fresh breadcrumbs, trim crusts from whole-wheat bread. Tear bread into pieces and process in a food processor until coarse crumbs form. One slice of bread makes about 1/2 cup fresh breadcrumbs.
To add extra flavor to the cream sauce, at the end of Step 3 stir in 1 tablespoon chopped fresh herbs, such as thyme, sage or parsley. Or make it cheesy by stirring in 1/2 cup shredded or crumbled cheese, such as Gruyère, Swiss, Cheddar or blue cheese.
Keep food fresh: If you're storing food in your fridge for a few hours or more, it's best to keep it in an airtight container or in a container covered tightly with foil. Foil is best at creating a barrier that doesn't let unwanted flavors in (or out) while you store your food.