01 -
Heat 2 tablespoons butter in a large, heavy nonstick skillet over medium-high heat until hot. Add the mushrooms, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Cook, stirring occasionally, until browned, about 6 minutes.
02 -
Wipe out the skillet. Heat the remaining 1 tablespoon butter in the skillet over high heat until hot. Sprinkle the beef generously with salt and pepper. Sear over medium-high heat, turning once halfway through, until browned, about 12 minutes for medium. Transfer the beef to a platter.
03 -
Add the shallots to the skillet and cook over medium heat, stirring, until golden, about 3 minutes. Add the mustard and heavy cream and bring to a boil, cooking until slightly thickened, about 3 minutes. Stir in the parsley.
04 -
Spoon the sauce onto a plate. Place the beef on the sauce and scatter the mushrooms over top. Garnish with additional parsley, if desired.