Bring a large pot of water to a boil over high heat. Salt the water. When the salt has dissolved, add the greens. Boil for 5 to 10 minutes, until the greens are cooked through and beyond the point of al dente; the amount of time will depend on the type of green and thickness of the stalks.
Drain and set aside to cool, about 20 minutes. When the greens are cool enough to handle, squeeze out any excess water. (To bring down the temperature of the greens faster, spread them out on a baking sheet. It will halve the cooling time.)
Heat the olive oil in a large skillet over medium heat. When the oil begins to shimmer, add the garlic and cook just until it begins to turn golden, about 5 minutes, then add the peperoncino.
Cook until fragrant, about 30 seconds, then add the boiled greens and turn to coat. Season with salt.
Cook, stirring frequently, for 8 to 10 minutes, until the greens have darkened and absorbed the garlic and peperoncino flavors.
Serve warm or at room temperature.