Put the lentils in a saucepan with the water over medium-high heat, and bring to a boil. Reduce the heat and simmer until the lentils are tender, about 15 to 20 minutes. Drain the water and reserve the lentils.
In a large bowl, whisk together the olive oil, lemon juice, vinegar, sugar, and season with salt and pepper, to taste. Stir in the lentils and leave them out to cool.
Meanwhile, blanch the green beans in a saucepan of boiling water until they are bright green and crisply tender. Rinse the beans under cold running water, and then drain well. Add the beans, celery and parsley to the lentils and toss the ingredients gently.
Recipe courtesy of William Granger