In a large pot of boiling salted water, blanch the green beans for 4 minutes, or until they are just crisp-tender. Drain the beans, transfer to a bowl of ice water to stop the cooking, and drain well. Put the green beans in a mixing bowl.
In a large skillet, cook the bacon over moderate heat, until crispy. Transfer the bacon with a slotted spoon to a plate lined with paper towels. Add the bacon to the green beans, along with the shallot, walnuts, and parsley.
In a mason jar, combine the mustard, water, sugar, and lemon juice; season with salt and pepper. Drizzle in the oil, put the cap on, and shake vigorously to emulsify. If planning to eat the salad right away, pour the dressing over the green bean salad and serve. If taking on a picnic, put the salad in a sealable plastic container and pack the dressing separately until ready to serve.