Bring a large pot of water to a boil, and salt it (for this, the same water may be used as that in which the potatoes have been blanched). Add the beans, and cook until they are just bright green and tender, about 5 minutes.
While the beans cook, fill a large bowl with ice water. Drain the beans, then plunge the colander into the water to stop the cooking. When they’re cool, drain them again. Zest the lemons, and julienne or mince the zest. Juice the lemons. (You can prepare the recipe to this point up to a day ahead; cover everything separately and refrigerate.)
When you’re ready to eat, put the oil in a large skillet over medium-high heat. Add the beans, and cook, tossing or stirring, until they are hot and glazed with the oil, 3 to 5 minutes. Toss the beans in a serving bowl with the lemon juice, top with the zest, and then serve.