Pat the sirloin dry with paper towels. If you are starting with a whole steak, slice it against the grain, then season the pieces with about 3/4 teaspoon of the kosher salt and all of the black pepper.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Sear the steak in a single layer for about 1–2 minutes per side, working in batches if necessary, then transfer it to a plate while it is still slightly underdone.

Add the remaining 1 tablespoon of olive oil to the skillet and add the sliced onion. Cook for 5–8 minutes, stirring occasionally, until the onion softens and develops golden edges.

Add the butter and sliced mushrooms. Cook for 5–7 minutes, allowing the mushrooms to release their moisture and begin to brown; add the remaining kosher salt after browning begins if desired.

Stir in the minced garlic and cook for about 30 seconds, just until fragrant. Avoid cooking the garlic too long, since it can turn bitter.

Pour in the beef broth, Worcestershire sauce, and Dijon mustard. Scrape the browned bits from the bottom of the pan and simmer for 2–3 minutes until the sauce smells rich and slightly reduces.

Return the sirloin and any accumulated juices to the skillet. Toss for 1–2 minutes, just until the beef reaches your desired doneness, then garnish with chopped parsley.