In a large saucepan, boil the potatoes in salted water until fork tender, then drain and allow them to cool slightly, for 10 to 15 minutes.
Meanwhile, in a separate pot, cook the carrots in slightly salted water until fork tender, then drain and plunge in a bath of icy cold water to stop the cooking.
Steam the beans for a couple of minutes, until tender but still slightly crunchy, then immediately plunge them in a bath of icy cold water to stop the cooking.
Drain the cooled beans and carrots.
While the vegetables are draining / cooling, make the vinaigrette: place all the ingredients in a glass measuring cup or similar container and mix well with a flat whisk until well combined and slighlty emulsified
Place the beans, carrots and potatoes in a very large salad bowl. Add the fresh parsley and vinaigrette and toss delicately until all the veggies are evenly coated.
This salad can be served immediately but it can also be made ahead of time and served cold.
It will keep well for up to 5 days in the refrigerator.
Notes
*Time saving tip: If you happen to have a steaming basket that sits nicely on top of a saucepan, like this model here, use that saucepan to cook the carrots and steam the beans at the same time.