Trim fat from chicken. Remove giblets and neck from chicken; reserve for another use. Rinse chicken under cold water; pat dry. Brush chicken with olive oil. Place chicken, breast side up, on a rack in a large roasting pan coated with cooking spray. Insert meat thermometer in meaty part of thigh, making sure it does not touch the bone.
Add potatoes to roasting pan. Bake, uncovered, at 350° for 1 hour. Add leeks and next 4 ingredients to roasting pan. Combine broth and next 4 ingredients; pour over chicken and vegetables. Bake 45 additional minutes or until meat thermometer registers 185°, basting occasionally with pan juices.
Transfer chicken and vegetables to a serving platter; keep warm. Pour pan juices through a wire-mesh strainer into a small saucepan, discarding solids. Combine flour and water, stirring until smooth; stir into pan juices. Cook over medium-high heat, stirring constantly, until thickened. Serve sauce with chicken and vegetables. Garnish with fresh thyme, if desired.