01 - Pat steaks completely dry with paper towels. Season both sides generously with kosher salt and freshly ground black pepper, pressing gently to adhere.
02 - Heat olive oil and 1 tablespoon butter in a large skillet over medium-high heat until butter foams. Add steaks and sear for 2 to 3 minutes per side for medium-rare, or until internal temperature reaches 130°F. Transfer steaks to a plate and tent loosely with foil to rest.
03 - Reduce heat to medium. Add remaining 2 tablespoons butter to the same skillet. Add sliced mushrooms and sauté for 3 to 4 minutes, stirring occasionally, until mushrooms release moisture and begin to brown.
04 - Add diced shallots and minced garlic to the mushrooms. Cook for 1 minute, stirring constantly, until shallots are translucent and garlic is fragrant. Do not let garlic brown.
05 - Stir in Dijon mustard until well combined. Carefully pour in brandy or cognac (away from heat source if using gas). Let simmer for 1 to 2 minutes, stirring occasionally, until alcohol reduces slightly and raw smell dissipates.
06 - Pour in beef broth, Worcestershire sauce, and heavy cream. Stir to combine and scrape up any browned bits from the bottom of the skillet. Simmer for 3 to 5 minutes, stirring occasionally, until sauce thickens enough to coat the back of a spoon.
07 - Return rested steaks and any accumulated juices to the skillet. Spoon sauce generously over steaks. Cook for 1 to 2 minutes just to heat through. Sprinkle with fresh parsley and serve immediately with extra sauce on the side.