Place the flour onto a flat plate. Season the escalopes well with salt and pepper. Dip the seasoned side into the flour.
Place a large frypan over a high heat. Fry the meat with the seasoned side down in hot oil. When the escalopes are nice and brown turn them over and fry the other side the same. This shouldn't take more than a couple of minutes per side. Set aside.
Reduce the heat a notch and melt in the butter. Add the onions and the mushrooms; season with salt and pepper and fry. Turn regularly so that the browned bits of the meat are removed from the bottom of the frypan.
Add the garlic and brown for a minute.
Deglaze everything with white wine and pour in beef stock. Mix well. If necessary add salt again to taste. Pour the sauce over the escalopes and serve immediately.