Heat the butter and oil in a large non-stick frying pan. Add the shallots and fry for 1 min.
Season the pork with freshly ground black pepper and add to the pan. Fry for 1 min on each side until well browned. Add the mushrooms and fry over a high heat for a further minute.
Add the wine and mustard and reduce the heat to a simmer. Cook for 10-12 mins until the pork is cooked through. Remove the pork and vegetables from the pan and cover and keep warm.
Boil the liquid in the pan until it's reduced by about half. Stir in the crème fraiche and heat through gently. Season to taste. Serve with French beans, a little of the sauce, and garnish with a few chopped chives.