In a flat dish combine the minced shallots and garlic together with the mustard, olive oil and herbs and mix well. Add the chicken breast fillets and toss to coat. Cover with cling film and chill in the fridge for 4 hours.
Remove the chicken from the fridge, wipe off any excess onion garlic mixture back into the dish. Melt the butter in a frying pan over a medium heat then add the chicken fillets and brown for about 4 minutes on each side. Remove from the pan and set aside.
Scrape the onion garlic mixture from the dish into the same frying pan and cook and stir for 5 minutes. Add in the apples and walnuts and continue to cook. Sprinkle in the flour and mix thoroughly then pour in the chicken stock and cream and simmer for 5 minutes.
Return the chicken to the pan and heat in the sauce for about 2 minutes, or until the chicken is no longer pink in the centre. Remove from the pan and serve with the sauce.