Put the green beans in a pan of boiling water and blanch them for 2-3 minutes until tender but still retaining a little bite. Refresh in iced water then drain and pat dry with kitchen paper. Place in a serving dish.
Pick over the herbs, tearing or chopping any larger pieces, then toss with the beans along with the thinly sliced shallot.
For the vinaigrette: whisk the vinegar, mustard, sugar and a pinch of sea salt in a small bowl until smooth. Gradually whisk in both the oils until the dressing becomes thickened, glossy and emulsified. Season with more salt, if necessary, and a couple of twists of black pepper. Dress the salad with the vinaigrette and serve.