Heat the butter and oil in a large heavy-bottomed skillet over medium-high heat.
Add the tournedos and cook for about 3 minutes per each side or until golden.
Place the tournedos in an ovenproof baking dish and keep warm in a preheated oven at 250°F (120°C).
Add the shallots, garlic and mushrooms to the skillet. Cook, stirring, for 2 minutes.
Add the thyme and green peppercorns and continue cooking, stirring, for another 2 minutes.
Add the vermouth (or white wine), scraping the bottom of the skillet with a wooden spoon to loosen the particles. Continue cooking until almost all of the liquid has evaporated.
Add the veal stock and simmer for 3 to 5 minutes or until the mixture has reduced by about half.
Stir in the cream and simmer for 2 to 3 minutes or until the sauce is slightly thickened. Season with salt & pepper.
Place the medallions on serving plates, drizzle with sauce and serve.