Step 1: Prep the Steak and Mushrooms
Pat the sirloin steak completely dry with paper towels. This is crucial for a proper sear. Cut the steak into uniform, 1-inch to 1.5-inch cubes. Season generously on all sides with kosher salt, black pepper, and garlic powder or smoked paprika if using. Let it sit at room temperature for 15-20 minutes. Meanwhile, clean your mushrooms by wiping them with a damp paper towel or a quick rinse and thorough dry. Quarter them. Mince the garlic and dice the shallot.

Step 2: Sear the Steak Bites – The First Blast of Flavor
Heat a large, heavy-bottomed skillet (cast iron is ideal) over medium-high to high heat until it is very hot. Add the high-smoke-point oil and swirl to coat. Working in batches to avoid overcrowding (which steams the meat), add the steak bites in a single layer with space between them. Do not move them for 2-3 minutes! This allows a deep, brown crust to form. Using tongs, flip each piece and sear for another 2-3 minutes for medium-rare, or to your desired doneness. Transfer the seared bites to a clean plate. Repeat with remaining batches.

Step 3: Sauté the Mushrooms – Building the Foundation
Without cleaning the skillet, reduce the heat to medium. Add 1 tablespoon of butter or oil. Add the quartered mushrooms in a single layer if possible. Let them cook undisturbed for 4-5 minutes to develop a good sear. Season with a pinch of salt and pepper. Stir and continue to cook for another 5-7 minutes, until the mushrooms have released their water, it has evaporated, and the mushrooms are deeply browned, shrunken, and tender. Transfer the mushrooms to the plate with the steak.

Step 4: Create the Aromatic Base
In the same skillet (now filled with glorious fond from both the steak and mushrooms), add the remaining 1 tablespoon of butter. Add the diced shallot and cook for 2-3 minutes until softened. Add the minced garlic and thyme, cooking for just 30-60 seconds until incredibly fragrant. Be careful not to burn the garlic.

Step 5: Deglaze and Build the Pan Sauce
Increase the heat to medium-high. Pour in the beef broth and red wine (if using), scraping the bottom of the pan vigorously with a wooden spoon to dissolve all the browned bits. Let it simmer for 2-3 minutes, allowing it to reduce by about half. Whisk in the Worcestershire sauce and Dijon mustard (if using).

Step 6: The Grand Reunion and Final Glaze
Reduce the heat to medium-low. Return the steak bites, mushrooms, and any accumulated juices on the plate back to the skillet. Toss everything together gently for 1-2 minutes, just until the steak is reheated and everything is coated in the glossy, reduced sauce. The sauce should cling to the meat and mushrooms beautifully.

Step 7: Finish and Serve
Remove the skillet from the heat. Taste and adjust seasoning with salt and pepper if needed. Garnish generously with freshly chopped parsley and a final pinch of flaky sea salt. For an extra touch of luxury, you can swirl in an additional tablespoon of cold butter off the heat to make the sauce even richer and glossier.

Step 8: Serve Immediately
Serve directly from the skillet or transfer to a warm serving platter. This dish is best enjoyed immediately while hot, ideally over or alongside something to soak up the incredible sauce—creamy mashed potatoes, buttery egg noodles, or a crusty loaf of bread are all perfect companions