Place the green beans in a large pot of salted boiling water. Cook the green beans until just crisp and a bit tender, about 3-4 minutes. Using a slotted spoon or small spider strainer remove the green beans from the boiling water and place into a bowl of cold water filled with ice cubes for a couple of minutes. Drain the green beans and set aside.

Heat the olive oil in a large skillet. Add the garlic slivers and cook for about a minute and then add the red pepper flakes, if using, and cook for another minute. Add the blanched green beans and cook until tender, about 5-8 minutes. Season the green beans with salt and pepper. Add a splash of sherry vinegar and stir for another minute or two. Serve hot or at room temperature.