Arrange the beef medallions on a plate and spread over the cracked mixed peppercorns, pressing down to form a thick crust. Season with salt.
Heat 15 g of the butter and the oil in a frying pan. Add the beef fillets and sauté for 3 minutes on each side for medium rare, or 6 minutes on each side for well done. Remove the beef fillets and set aside to rest in a warm place.
Add the shallot to the pan and cook over a low heat for 30 seconds, stirring continuously. Do not allow the shallots to colour at all. De-glaze the pan with the Cognac and sherry vinegar, stirring well to scrape up the meat juices. Add the reduced beef stock and cream, bring to the boil and then reduce for a few minutes until the sauce coats the back of a spoon.
Add the 25 g of the remaining butter to a large pan and once it starts to foam tip in the spinach. Sauté over a fairly high heat until just wilted. Season to taste and drain off any excess liquid. Place the remaining 25 g of butter in a frying pan and gently sauté the wild mushrooms for a few minutes until tender, then season to taste and gently mix with the spinach.
Spoon the sautéed spinach and wild mushrooms on to serving plates. Arrange the steak medallions on top, then spoon the sauce around to serve.