Peel a 2-inch-long strip of zest from the lemon; tie into a bundle with the parsley and thyme sprigs and bay leaf using kitchen twine. Put in a saucepan with the garlic, potatoes, 2 tablespoons salt and 8 cups water. Bring to a boil, then reduce to a simmer and cook about 10 minutes. Add the carrots and cook until the potatoes are tender and the carrots are crisp-tender, about 5 more minutes. Reserve 1/4 cup of the cooking water, then drain the vegetables and discard the herb bundle. Let cool. Meanwhile, finely grate 2 teaspoons lemon zest and squeeze 3 tablespoons juice into a large bowl. Whisk in the yogurt, mayonnaise, mustard, scallion whites, chopped parsley, reserved cooking water, 1 teaspoon salt and 3/4 teaspoon pepper. Add the potatoes, carrots, celery and cucumber to the dressing and toss. Chill about 4 hours. Top with the scallion greens.