Boil or steam the broccoli raab for about 3 minutes, until it is bright green and beginning to get tender. Drain and plunge it into ice water for a few moments and drain again. (You can prepare the broccoli raab up to this point, wrap it well or place in a covered container, and refrigerate for up to 2 days.) Squeeze the excess liquid from the broccoli raab and chop it coarsely.
Heat the oil, chili flakes, and garlic in a large skillet over medium heat until the garlic is fragrant. Add the broccoli raab and cook, stirring occasionally, until it’s heated through, 5 to 7 minutes. Remove from the heat, season with salt and pepper to taste.