Bring a large pot of salted water to a boil. You will need to boil the onion first, the green beans second and the corn last, all in the same pot of water, so make sure you will have a strainer handy or a steamer basket to remove the onions.

Boil the onions for 1 minute. Using the strainer, transfer the onion slices to an ice bath and chill for about 2 minutes. Remove the onion slices and place on a paper toweled bowl.

Boil the green beans until tender, about 5 minutes. Transfer them to the ice bath and chill for 3 to 4 minutes. Remove the beans to a paper lined bowl.

Boil the corn for 2 minutes, and then immerse the ears in the ice bath until completely chilled, about 5 minutes. Add more ice cubes to the ice bath as needed.

In a blender, combine the shallots, the ¼ cup vinegar, a nice pinch of salt, and several grinds of pepper. . Process briefly then with the motor running adds the ½ cup olive oil.

Cut the corn off the cob, cutting deeply enough against the cob so that the corn comes in planks. Remove the paper from the bowl of the green beans. Toss the green beans with half of the vinaigrette. Top with the plank of corn and scatter any remaining kernels around the salad.