Advice from my brother who has done a few miles there:
Talavera de La Reims, Gredos, Segovia or Salamanca, Burgo and Reinosa, Pamplona and Pyrennes. Not the most direct but enjoyable.
Or any green road on the Michelin map.
The French bit I've done but just following the yellow D roads up through the country, found some great roads, small towns and villages but not recorded exact routes.
Talavera de La Reims, Gredos, Segovia or Salamanca, Burgo and Reinosa, Pamplona and Pyrennes. Not the most direct but enjoyable.
Or any green road on the Michelin map.
The French bit I've done but just following the yellow D roads up through the country, found some great roads, small towns and villages but not recorded exact routes.

