Wash rapini (see recipe note #1); drain and coarsely chop.
Heat a large pot of salted water to boil. Add rapini and cook 1 minute (working in batches if necessary); transfer rapini to colander to drain.
Heat oil in a large skillet over medium heat. Add garlic and red pepper flakes; cook and stir until garlic is fragrant and golden, 1-2 minutes. Add rapini and raisins; cook and stir until raisins are plump and rapini is tender, 4-5 minutes. Season to taste with kosher salt and transfer to platter. Sprinkle with hazelnuts and serve.
To wash rapini, submerge in a large bowl of water. Swoosh it around to dislodge all dirt and debris. Lift out rapini and transfer to a colander to drain.
For a fresh take on this dish, feel free to swap in dried cherries or dried cranberries for the raisins and/or toasted pine nuts for the hazelnuts.