In large pot of boiling salted water, blanch celery until tender-crisp, about 30 seconds. Remove with slotted spoon; chill under cold water. Drain and transfer to large bowl.

Cook potatoes in same pot until tender, 25 minutes. Drain and let cool; cut in half and add to bowl. Mix dressing with tarragon. Add ½ cup (125 mL) of the dressing mixture to bowl; toss to coat.

Meanwhile, brush chicken with 1 tbsp (15 mL) oil; sprinkle with ¼ tsp (1 mL) each salt and pepper. Place on greased grill over medium-high heat. Close lid and grill, turning once, until no longer pink inside, about 12 minutes. Transfer to cutting board; tent with foil and let stand for 5 minutes.

Slice chicken. Spoon potato salad onto plates. Top with chicken; drizzle with remaining dressing.

Makes 4 servings.