Place peppercorns in sturdy plastic bag and seal. With small skillet or heavy saucepan, pound to crush peppercorns coarsely. Spread on plate. Pat steaks dry; press evenly into peppercorns. Set aside.
Brush large heavy skillet with oil; cook steaks over medium-high heat, turning once, until well browned on both sides and rare inside, 6 to 8 minutes, or until medium, 12 to 15 minutes. Transfer to warmed plate; tent with foil and let stand for 5 minutes. Wipe out any peppercorns stuck to pan.
Drain off any fat from skillet and return to medium-high heat; cook butter and mushrooms, stirring often, until mushrooms are tender, very golden and liquid has evaporated.
Warm brandy in small saucepan over low heat; ignite with long match. Pour over mushrooms. When flames die down, pour in cream; bring to boil. Boil until slightly thickened, about 2 minutes. Stir in mustard; heat through.
To serve, slice steaks across the grain; fan over warmed plates. Ladle sauce over top.