Preheat the oven to 180c. Wrap the chicken breasts in foil and place in the oven for 35-45mins, depending on the thickness of the chicken.
In the meantime, melt the butter in a large lidded frying pan, add the olive oil, mushrooms and onions and over a medium heat fry until the mushrooms and onions until soft.
Add the garlic and stir for another few mins.
Add the sherry and cook until the liquid is reduced by half.
Add the stock and bay leaf and gently simmer for around 15mins.
When the chicken is cooked, remove from the oven and set aside.
Add the juices from the chicken (optional), cream and crème fraiche, stir together and then add the chicken breasts, pepper and simmer for around 10mins.
Serve immediately with some plain boiled rice or potatoes (hasselbacks would go great with this dinner!)
