Step 1Wash the spigarello thoroughly. Pull the leaves off the stems, grabbing the leaves from the top of the stem and pulling toward the bottom to remove (in the same way you'd strip rosemary needles). Discard the stems. Bring a large pot of salted water to a boil. Add the spigarello, return to a boil and blanch for 3 to 5 minutes, until the greens wilt slightly and bend easily in the pan. Drain in a colander.
Step 2Heat the olive oil in a large heavy skillet over medium-high heat. Add the spigarello, season with salt and saute until slightly crispy, about 3 to 5 minutes.
Step 3Stir in the garlic and red pepper flakes, turn off the heat and let stand for about 1 minute, stirring often. (If the pan is too thin to hold much heat, cook over low heat for 1 minute after adding the garlic and pepper flakes.) Sprinkle with vinegar and toss lightly. Serve warm.