Adjust oven rack to lowest position and heat oven to 425 degrees. Whisk yogurt, lemon zest and juice, and pinch salt together in bowl. In separate bowl, combine sesame seeds, coriander seeds, thyme, and pinch salt. Set both aside until ready to serve.
Toss celery root with oil, 1/2 teaspoon salt, and 1/4 teaspoon pepper and arrange on rimmed baking sheet in single layer. Roast celery root until bottom sides toward back of oven are well browned, 25 to 30 minutes. Rotate baking sheet and continue to roast until bottom sides toward back of oven are well browned, 6 to 10 minutes.
Use metal spatula to flip each piece and continue to roast until celery root is very tender and other sides are browned, 10 to 15 minutes. Transfer celery root to serving platter. Drizzle celery root with yogurt sauce and sprinkle with seed mixture and cilantro. Serve.