Heat oven to 425°F. Coat 2 large baking sheets with cooking spray.
Beat egg white slightly in large bowl. Stir in thyme, salt and pepper. Add all-purpose potatoes and sweet potatoes; toss to coat. Sprinkle stuffing crumbs over potatoes; toss to coat. Place in single layer on prepared baking sheet.
Bake in 425°F oven 10 minutes.
While potatoes are baking, stir together yogurt, mustard, lemon juice, salt and pepper in large bowl. Add chicken; turn to coat.
Combine stuffing crumbs and thyme in pie plate. Add chicken, one piece at a time; roll in crumbs to stick. Transfer to prepared baking sheet. Lightly coat chicken with cooking spray.
After potatoes have cooked 10 minutes, add chicken to 425°F oven. Bake both potatoes and chicken 20 minutes or until chicken is golden brown and potatoes are crispy.
To crisp chicken if desired, broil for 3 minutes.