Cut a slit into the thickest part of each chicken breast to make a pocket.
Heat the butter and oil in a shallow frying pan. Add the chicken breast and cook till browned.
Remove from the pan and cool a little.
Mix together the breadcrumbs, thyme, lemon juice and zest and Saint Agur. Stuff a quarter of the mixture into each chicken breast.
Place in a shallow baking dish and cook covered for 20 minutes at 190ºC/375ºF/Gas mark 5. Remove the cover and cook for a further 5-10 minutes.
Cut each chicken breast into 4-5 slices and serve on a bed of watercress and baby spinach.