Make the mushroom sauce. Sauté the mushrooms and shallot in 20g butter over a medium heat until just cooked. Add the wine, and simmer to reduce by half. Add the stock, bring to the boil, then reduce by half again. Stir in the cream, bring to the boil, then sieve into a small pan. Season with salt and freshly ground black pepper and keep warm.
Make the mustard cream sauce by whipping the cream until it just holds its shape, then stir in the mustards and season.
Boil the potatoes until soft, then drain. Return them to the pan and stir in 60g of the butter and a generous sprinkling of salt. Garnish with half the parsley.
While the potatoes are cooking, melt 30g of butter with the oil in a frying pan over a high heat, then sear the fillets on both sides. Turn the heat down low, cover the pan and cook gently for 5 to 15 minutes (depending on size of the steaks).
While the beef is cooking, sauté the mushrooms in 20g butter.
Place the potatoes, mushrooms and meat on warmed plates. Drizzle with mushroom sauce and garnish with mustard cream and remaining parsley.