Heat half the butter and the oil in a frying pan, add the chicken breasts and fry briefly until browned. Put in the slow cooker pot.

Add the onion to the pan and fry for 5 mins. Add remaining butter and mushrooms and fry for 3 mins.

Pour in the soup and bring to the boil. Add thyme to slow cooker pot and pour the soup mixture over the top.

Cover with lid and cook on low for 7-8 hours. Test the chicken. Serve.