Season the chicken on both sides with some salt and freshly ground black pepper. Heat a few sprays of the olive oil in a non-stick frying pan, shallow casserole or a skillet pan on a medium heat, add the seasoned chicken breasts and fry for about 10 minutes (turning halfway through), or until well browned on both sides. Set aside on a plate.
In the same pan, add a little more spray olive oil and fry the onion for 2 minutes, or until translucent. Add the garlic and fry for 1 minute, then add the mushrooms and fry until golden brown (approx. 3 to 4 minutes). If they start to stick, add a little stock to the pan.
Return the chicken to the pan and stir in the stock. Bring to the boil. Cover with a tight-fitting lid, lower the heat and simmer for 12 to 15 minutes, or until the chicken is cooked through (you can check that the internal temperature is 74C by using a meat thermometer).
Remove the pan from the heat and transfer the chicken to a plate. Stir the crème fraiche into the sauce, then return the pan to a medium high heat. Once it starts to bubble continuously, slowly pour in the cornflour slurry, stirring the sauce as you do. You may not need it all – use as much as you need until the sauce has thickened slightly.
Stir the parsley into the sauce then season, to taste, with some salt and freshly ground black pepper. Add the chicken back into the pan and let it warm through for a couple of minutes.
Serve the chicken and sauce with some basmati rice, pasta, mashed potatoes and extra vegetables on the side.