Cut the 4 chicken breasts into thin strips. Heat 2 tablespoons oil in a deep pan over medium-high heat. Fry the chicken for 5-8 minutes, stirring occasionally, until golden brown. Remove to a plate and cover with foil.
Peel the 2 apples, remove the cores, and cut into small wedges.
Heat the 30 g butter in the same pan over medium heat until foaming. Add the apple wedges and 2 teaspoons brown sugar. Fry for 3-5 minutes, stirring constantly, until the apples are golden yellow but not mushy.
Add the 2 tablespoons calvados and stir for 1-2 minutes until the liquid evaporates.
Stir in the 300 ml cream and 2 teaspoons grainy mustard. Return the cooked chicken to the pan and bring the mixture to a boil over medium-high heat. Immediately reduce the heat to medium and simmer for 3 minutes.