Heat a small amount of oil in a pan until the pan is quite hot. Season steaks on both sides with salt and pepper. Place in pan sear 2-3 minutes each side. For medium rare cook for a further 2-3 mins on each side.
Of course longer cooking times apply for more well done meat and it does depend on how thick the meat is.
For the Sauce.
Melt butter in a pan and saute the shallots and garlic until soft add Worcestershire and mushrooms continue to cook over medium heat until mushrooms start to soften.
Add the mustard, lemon juice, beef stock and brandy, stir to combine, cook for 2-3 minutes add both the creams and thyme. You want to cook until sauce reduces slightly and thickens. if the sauce becomes to thick just add a little more of the half fat cream.
Finally add chopped parsley and chives.
Now you can either be cooking the steaks at the same time as making the sauce or it is fine to leave on a low heat while you cook the steaks just stir occasionally.