Place the potatoes in a large saucepan and fill with cold water. Add a good pinch of kosher salt and bring to a boil over high heat. Reduce the heat a bit and boil until the potatoes are tender.

Add the haricot verts and boil for another 2-3 minutes, until the beans are crisp-tender. Strain the potatoes and beans out and plunge them into a large bowl of ice water to stop the cooking.

In a large bowl, stir together all of the dressing ingredients. Add the cooled, drained potatoes and haricot verts along with tomatoes and green onions. Stir well to coat everything in the dressing and taste for seasoning, adding more salt and pepper if necessary.

Cover the bowl and place in the refrigerator to chill until ready to serve.