Remove the leaves from the rapini stems. Reserve the leaves for another use.
Prepare an ice bath. Bring a large pot of salted water to a boil. Working in batches, blanch the rapini just long enough to set the color and tame the bitterness, about 1 minute, then transfer to the ice bath to cool. Repeat with the remaining rapini, then drain and squeeze the rapini dry with a clean kitchen towel.
In a large skillet, warm the olive oil over medium-low heat. Add the garlic and red-pepper flakes and cook until fragrant, about 1 minute. Add the rapini and cook, stirring occasionally, until the rapini is warmed through but still slightly crunchy, about 4 minutes.
Remove the skillet from the heat. Squeeze the lemon over the rapini and toss gently to combine. Season with salt and pepper and serve immediately.