Drizzle olive oil over the steaks and season generously with salt and freshly cracked black pepper. Press and roll the steaks in the seasoning to make sure the steaks are covered on all sides.
Heat a large pan over medium-high heat. Add a splash of oil to the pan then add the steak. Allow to cook for 2-3 minutes per side until well-browned.
Add the butter, whole garlic cloves and fresh thyme.
Baste the steak with the butter until it is cooked to your liking. Medium rare: 55ºC/135ºF. Medium: 63º/145ºF.
Transfer steaks to a plate and pour over the butter and aromatics. Allow to rest while you make the sauce.
Add the onion/shallot and garlic to the pan and cook until soft and translucent.
Pour in the beef stock (you can use brandy as well if you prefer) then allow to simmer, scraping the browned bits off the bottom of the pan.
Pour in the cream and stir in the mustard, lemon juice and cracked pepper. Bring to a simmer and cook for 5 minutes until the sauce has reduced slightly and coats the back of a spoon.