Remove steak from the refrigerator 20-30 minutes before cooking to bring it closer to room temperature. Pat it thoroughly dry with paper towels. Season generously on all sides with coarse sea salt and freshly ground black pepper.

Heat olive oil in a large cast iron skillet over high heat until just smoking. Carefully place the seasoned steak in the hot skillet. Sear for 2-4 minutes per side for medium-rare, or longer for desired doneness, developing a deep brown crust.

Transfer the seared steak to a cutting board and tent loosely with foil. Allow it to rest for at least 5-10 minutes while you prepare the mushrooms. This helps the juices redistribute.

Reduce the skillet heat to medium. Add butter to the same skillet. Once melted, add the sliced mushrooms. Sauté, stirring occasionally, until the mushrooms are tender and have released their liquid, about 5-7 minutes.

Add minced garlic to the skillet with the mushrooms and cook for 1 minute until fragrant. Pour in the beef broth and bring to a simmer, scraping up any browned bits from the bottom of the pan. Let it reduce slightly for 2-3 minutes until it thickens a bit. Stir in fresh thyme if using.

Slice the rested steak against the grain into desired thickness. Return the sliced steak to the skillet with the mushrooms and sauce, tossing gently to coat. Garnish with fresh chopped parsley and serve immediately.