Cut the chicken breast fillets into thin strips and fry them in the heated oil in a deep pan until golden brown. Set aside and cover with foil.
Peel the apples, remove the core and cut into small wedges. Froth the butter in the pan, add the apple wedges and the brown sugar. Fry for about 3 - 5 minutes, stirring constantly, until the apples are golden yellow but not yet mushy. Add the calvados and stir until the liquid has evaporated. Stir in the cream and mustard. Put the chicken back in the pan and bring everything to the boil. Immediately turn down the temperature and simmer on medium level for another 3 minutes. Season to taste with salt and pepper.