Instructions

• Preheat the broiler to high. Line a rimmed baking sheet with foil.
• Lightly score shallow cuts on both sides of the 10ouncestop sirloin (if the sirloin is in smaller pieces, score each piece).
• Sprinkle both sides of the steak generously with freshly ground black pepper and salt.
• Place the steak on the prepared baking sheet and broil 4 to 6 minutes per side, until the steak reaches your desired level of doneness.
• Remove the steak from the oven, tent loosely with foil, and let rest for at least 5 minutes before slicing.
• While the steak cooks, heat a medium skillet over medium heat.
• Add the 1 cup sliced mushrooms and the thinly sliced green onion to the skillet. Sauté, stirring occasionally, until the mushrooms are lightly browned, about 3 to 4 minutes. If they begin to stick, add a small splash of the beef broth.
• Add the diced clove of garlic and cook, stirring, for about 30 seconds. Transfer the vegetables to a plate and return the pan to the heat.
• Pour 1 cup beef broth into the pan and bring to a simmer. Simmer 3 to 4 minutes, until the broth is reduced by about half.
• Remove the pan from the heat and whisk in 2 tablespoons sour cream and 1 tablespoon Dijon mustard until smooth. Taste and add salt and freshly ground black pepper as needed (if you used full-sodium beef broth, you may not need additional salt).
• Keep the sauce warm over very low heat if not serving immediately—do not boil.
• Slice the rested steak across the grain. Serve the sliced steak with steamed green beans and spoon the mustard sauce over the steak and beans.