How to Make It
Preheat the oven to 400° and grease 2 large rimmed baking sheets with olive oil. In a large bowl, combine the potatoes, 1/4 cup of the olive oil and the thyme. Season generously with salt and pepper and toss well. Spread the potatoes in a single layer on the prepared baking sheets. Bake for about 15 minutes, until browned on the bottom. Using a thin spatula, flip the potatoes and bake for 15 minutes longer, until they are crisp. Sprinkle with salt and let cool completely.
Meanwhile, in a large saucepan of salted boiling water, blanch the haricots verts until crisp-tender, 2 to 3 minutes. Drain and cool under running water. Pat dry with paper towels, then cut the beans in half.
In a large serving bowl, whisk the shallot with the vinegar and a pinch of salt and let stand for 5 minutes. Whisk in the mustard and the remaining 3 tablespoons of olive oil. Add the haricots verts and parsley, season generously with salt and pepper and toss to coat. Scatter the crisp potatoes on top before serving.