Preheat oven to 450 degrees. On a rimmed baking sheet, drizzle broccoli rabe with olive oil and sprinkle with garlic and pepper flakes. Season with salt. Toss by hand, massaging oil into greens. Spread in a single layer.
Roast 10 minutes. Remove from oven; toss with sesame oil. Return to oven and cook until just tender and leaves are crisp, about 5 minutes more. Serve warm or at room temperature, atop crostini or as a side dish.