1. Heat 2 tablespoons butter in a large, heavy nonstick skillet over medium-high heat until hot. Add the mushrooms and 1/2 teaspoon salt and 1/2 teaspoon pepper. Cook, stirring occasionally, until browned, about 6 minutes.
2. 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.
3. 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. Spoon the sauce on a plate; place the beef on the sauce and scatter the mushrooms over top. Garnish with additional parsley, if desired.
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