Coat 2 large baking sheets with cooking spray. Beat egg whites slightly. Stir in 1 tsp. thyme, 1/2 tsp. salt and 1/4 tsp. pepper. Add potatoes and toss to coat. Toss in stuffing crumbs and place in single layer on baking sheet. Bake at 425F for 10 min. Stir together yogurt, Mustard, lemon juice, 1/2 tsp. salt and 1/4 tsp. pepper. Combine stuffing and 1 1/2 tsp. Thyme. Coat chicken and toss in stuffing crumbs. Transfer to baking sheet. After potatoes have baked 10 min. turn potatoes over and return to oven. Add chicken and bake both another 20 min.