Heat the oven to 500ºF with an oven rack in the middle position. Line a rimmed baking sheet with foil. In a large bowl, toss the broccoli rabe with the oil, fennel, salt, black pepper and red pepper flakes. Transfer the broccoli rabe to the baking sheet, reserving the bowl, and roast until just beginning to brown, stirring halfway through, 12 to 15 minutes. Let cool. Return to the bowl and toss with the dressing.