Bring a large pot of salted water to a boil. Prepare a large bowl of ice water, and place a colander inside bowl. Line a baking sheet with a double layer of paper towels. Cook green beans in boiling water until bright green and crisp-tender, 2 to 3 minutes. With a wire skimmer or slotted spoon, transfer to colander to stop cooking. Cool green beans completely in ice bath, then transfer to lined baking sheet; pat dry.
In a small saucepan, heat vegetable oil over medium-low. In a small bowl, toss shallots with flour. Working in three batches, fry shallots in oil until golden and crispy, 2 to 4 minutes. Transfer shallots to paper towels, and season generously with salt.
In a small bowl, whisk lemon juice, mustard, and olive oil to combine; season dressing with salt and pepper. Place watercress on a serving platter, and drizzle with half the dressing. Top with green beans and remaining dressing. Serve sprinkled with fried shallots.