In a large skillet, melt the butter in the olive oil. Season the meat with salt and pepper and cook over high heat until lightly browned on the bottom, about 1 minute. Turn the medallions and cook for 45 seconds longer, then transfer to a plate and tent with foil.

Add the shallot and garlic to the skillet and cook over moderate heat, stirring, until fragrant, about 20 seconds. Add the mushrooms, season with salt and pepper and cook until softened, about 2 minutes. Remove from the heat, add the Cognac and carefully ignite it with a long match. When the flames die down, add the mustard and cream and stir over moderate heat for 1 minute. Whisk in the veal demiglace or concentrated stock and Worcestershire sauce. Check for seasoning.

Serve with wilted greens and mashed potatoes, if desired.