Position racks in upper and lower third of oven; preheat to 425°F.

Toss potatoes with oil in a large bowl until well coated. Divide between 2 large rimmed 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.

Meanwhile, cook bacon in a large saucepan over medium heat, stirring frequently, until just crisp, about 3 minutes. Remove the bacon with a slotted spoon to a paper towel-lined plate.

Add onion to the pan; 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; 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 heat.

When the potatoes are done, remove them 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 and top with 1/4 cup cheese and 2 tablespoons chives. Add the remaining potatoes, sauce, and cheese. Top with the bacon.

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 for 10 minutes. Serve topped with the remaining 2 tablespoons chives.