This can be done with any fortified wine; I prefer Madeira.

Pound the steak with a rolling pin to flatten slightly and cut into serving pieces. Crush the peppercorns with the rolling pin and spread over both sides of the meat. Salt to taste. (The pepper may be applied in advance, but the salt should wait until cooking time.)

Melt two tablespoons of butter in the oil in a large pan over medium-high heat. Sauté the meat on both sides to medium-rare (in batches if necessary) and remove to a serving platter. Deglaze the pan with the wine, mix in the mustard and cream, and reduce to thicken. Turn off the heat and swirl in the remaining butter. Pour the sauce over the meat and serve.