Place the green beans in a steamer basket set over a large saucepan of boiling water. Steam until bright green and crisp-tender, about 5 minutes.
In a medium skillet, heat the oil. Add the shallot and cook over high heat, stirring, until softened, about 2 minutes. Add the vinegar and sugar; stir to dissolve the sugar. Remove from the heat and add the beans. Season with salt and pepper, toss well and serve.