Rub the steaks with the oil and season generously with salt and pepper. Put a large non-stick or heavy cast iron pan over a very high heat until smoking hot. Carefully lay the steaks in the pan, then add the butter on the side along with the thyme. Sear the steaks on each side, depending on their thickness and how well you want them cooked (1-2 mins for rare, 3-4 mins for medium and 5-6 mins for well done). Baste them with the foaming butter and thyme as they sear, then remove from the pan and leave to rest on a warm plate or board that has a gutter to catch the juices.
To make the sauce, put the pan back on a medium heat with the fat from the steak and the thyme, and add the butter. Once melted and sizzling, add the shallots and fry for 2 mins, then add the garlic and mushrooms, turn up the heat and fry for 2-3 mins more until the mushrooms have softened. Splash in the brandy and allow the liquid to boil and reduce for 1 min. Stir in the Worcestershire sauce, then pour in the stock. Bring to the boil and cook for 2 mins until reduced by half. Stir in the cream or crème fraîche and simmer until rich and creamy. Taste and add more salt if needed. Scoop out the thyme, then return the steaks and any juices to the pan and spoon over the sauce. Sprinkle over the parsley, if using. Bring the steaks to the table in the pan and serve drizzled with more of the sauce and a big bowl of chips or buttery mash on the side.