Prepare a grill or grill pan for medium-high heat.
Place the broccoli rabe on a sheet of aluminum foil that is large enough to fold over and seal in the rabe. Drizzle the broccoli rabe with the olive oil and sprinkle with the salt and red pepper flakes. With your hands, toss gently to coat. Fold the foil over the broccoli rabe and crimp all sides to seal. Place the sealed packet on the grill and allow to steam for 8 minutes.
Open the packet, being careful not to burn yourself with the steam that escapes. Using tongs, place the broccoli rabe directly on the grill. Cook on the first side until lightly charred in spots, about 2 minutes, then turn over and cook for an additional 2 minutes. Remove to a platter to serve.