Remove the steaks from the refrigerator 30 minutes before cooking and pat dry with paper towels. Season all sides with salt, pepper, and garlic powder.
Heat the cast iron skillet over high heat for 2 minutes until very hot. Add the olive oil and swirl to coat.
Place the steaks in the skillet and sear without moving for 3 to 4 minutes until a deep brown crust forms on the bottom.
Flip the steaks, add the butter, smashed garlic, and thyme sprigs to the pan, and cook for 2 to 3 more minutes, tilting the pan and spooning the melted butter over the top of each steak continuously.
Check the internal temperature. For medium-rare, remove the steaks at 130°F (54°C); for medium, remove at 140°F (60°C). The temperature will rise by about 5°F during resting.
Transfer the steaks to a cutting board and rest for 5 to 8 minutes, tented loosely with foil.
While the steaks rest, prepare the mushroom sauce. Reduce the heat to medium and add the 2 tablespoons butter to the same skillet.
Add the sliced mushrooms and cook for 5 to 6 minutes without stirring for the first 2 minutes, then tossing occasionally, until the mushrooms are golden brown and have released and reabsorbed their moisture.
Add the minced garlic and thyme and cook for 1 minute, stirring.
Pour in the white wine and scrape up any browned bits from the bottom of the pan. Let it reduce for 1 minute.
Add the beef broth and Worcestershire sauce and simmer for 3 to 4 minutes until slightly reduced.
Stir in the heavy cream and simmer for 2 to 3 more minutes until the sauce has thickened enough to coat the back of a spoon. Taste and adjust salt and pepper.
Plate the steaks and spoon the mushroom sauce generously over the top. Serve immediately.