Heat olive oil in a large frying pan over medium heat.
Season chicken breasts with salt and black pepper.
Brown the chicken for 4–5 minutes per side until golden.
Remove chicken and set aside.
Add onion, leeks, and mushrooms to the pan and cook for 5 minutes.
Stir in the sliced apples and cook for another 2 minutes.
Pour in the cider and chicken stock and bring to a gentle simmer.
Return the chicken to the pan and sprinkle over the thyme.
Cover and cook for 20–25 minutes until the chicken is fully cooked.
Stir in the Dijon mustard and half-fat crème fraîche.
Simmer gently for 2–3 minutes without boiling.
Garnish with parsley and serve hot.