Preheat oven to 450F. Thinly slice the white and light green part of the leek. Rinse leek slices well to remove any grit, then pat dry. Trim and cut in half the Brussels sprouts.
Chop the pancetta. Combine the leeks with the Brussels sprouts, pancetta, sage, oil, salt and pepper in a large roasting pan. Roast, stirring once, until the Brussels sprouts are tender, 18 to 20 minutes.