Step 1Combine the mustard and vinegar in a small bowl and whisk to blend. Whisk in the hazelnut oil until emulsified. Add the shallot, cornichons, chives and tarragon and whisk again. Season with salt and pepper to taste. Set aside.
Step 2Combine the bay leaf, garlic and sea salt in a large pot and add 3 quarts water. Bring to a boil. Add the haricots verts and blanch until tender, 3 to 5 minutes. Remove the beans with a slotted spoon and pat dry. Add the potatoes to the water and boil until tender, 15 to 20 minutes. Drain, reserving 2 tablespoons cooking water. Cool the potatoes for about 10 minutes, then slice them into one-fourth-inch slices. Cut the haricots verts into 1-inch pieces. Combine in a large bowl.
Step 3Add the reserved cooking water to the vinaigrette and whisk to blend. Pour the dressing over the potato mixture and toss with rubber spatula to coat completely. Season with salt and more pepper if needed.
Step 4Divide the mesclun among 4 serving plates. Divide the potato-bean mixture among the plates, spooning it onto the greens. Cut the duck into thin slices, then crosswise into half-inch-wide strips. Strew the duck slivers over the potato mixture. Arrange the tomato halves around perimeter. Sprinkle with toasted hazelnuts and serve.