Pat the sirloin steaks dry thoroughly with paper towels. Season generously on both sides with salt and freshly ground black pepper. Allow them to sit at room temperature for at least 15 minutes before cooking; this helps them cook more evenly and develop a better crust.

Heat 1 tablespoon of olive oil in a heavy-bottomed skillet (preferably cast iron) over medium-high heat until shimmering and almost smoking. Carefully place the seasoned steaks in the hot skillet. Sear for 3-4 minutes per side for medium-rare (internal temperature 130-135°F), adjusting time for your desired doneness.

For the last minute of cooking, add 1 tablespoon of unsalted butter, the smashed garlic cloves, and rosemary sprigs to the skillet. Tilt the pan and spoon the melted butter mixture over the steaks repeatedly to baste, enhancing their flavor and crust.

Remove the cooked steaks from the skillet and transfer them to a cutting board. Tent loosely with aluminum foil and let them rest for 5-10 minutes. This crucial step allows the juices to redistribute, ensuring juicy and tender steaks. Do not clean the skillet; you will use it for the sauce.

Return the same skillet to medium heat (no need to clean, those browned bits are flavor!). Add 1 tablespoon of olive oil. Once hot, add the sliced cremini mushrooms. Sauté, stirring occasionally, until they release their liquid and turn golden brown, about 5-7 minutes.

Add the minced garlic to the mushrooms and cook for another minute until fragrant, being careful not to burn the garlic.

Pour in the dry white wine (or beef broth). Scrape the bottom of the pan with a wooden spoon or spatula to loosen any browned bits (fond). Let it simmer and reduce by half, about 2-3 minutes.

Stir in the heavy cream, beef broth, and Dijon mustard. Bring the sauce to a gentle simmer and cook, stirring occasionally, until it thickens slightly to a consistency that coats the back of a spoon, about 5-7 minutes.

Remove the sauce from heat. Stir in the chopped fresh parsley. Taste and season with additional salt and black pepper as needed.

Slice the rested steaks against the grain into thick strips. Arrange on plates and generously spoon the creamy garlic mushroom sauce over the top. Serve immediately with your favorite complementary sides.