Place veal between two sheets of cling wrap and beat them with a rolling pin or mallet or I've even used a small frypan until very thin (if they're already paper thin when you buy them, don't bother with this step).
Heat the olive oil and 1 tablespoon of the butter in a large frying pan/skillet over high heat and cook the veal quickly, only about 1 minute on each side, in batches so you don't overcrowd the pan and set aside on a plate and cover loosely with foil to keep warm.
Into the same pan add the remaining butter and the garlic and mushroomns and saute until the mushrooms are browned.
Deglaze the pan with the wine and simmer until it is almost evaporated and then add the stock, cream and mustard and siommer until the sauce reduces slightly and starts to thicken.
Season to taste with salt and pepper and then pour over the veal, sprinkle with ground black pepper, if you like.