Bring a large pot of salted water to a boil. Add half of the greens and cook, stirring a few times, until just tender, about 3 minutes. Using a slotted spoon, transfer the greens to a colander and rinse with cold water. Repeat with the remaining greens. Drain very well. Coarsely chop the greens.

In a large, deep skillet, heat the olive oil. Add the garlic and crushed red pepper and cook over moderate heat until the garlic is golden brown, about 1 minute. Add the greens and stir well. Cover and cook, stirring occasionally, until the greens are heated through, about 8 minutes. Season with salt and pepper. Transfer the greens to a large bowl and let cool to room temperature. Stir in the vinegar and serve.

The cooked greens can be refrigerated overnight. Serve lightly chilled.