French-Fried Onions

Preheat oven to 400 degrees F. Combine onion and buttermilk in a medium bowl. Let stand for 15 minutes. Drain the onion (discard buttermilk) and return to the bowl. Sprinkle with flour, pepper and salt; toss to coat. Transfer to a large rimmed baking sheet. Coat generously with cooking spray. Bake, stirring once or twice, until tender and crisp, 25 to 28 minutes. If desired, let the onions stand at room temperature for up to 2 hours.