Aside from the steak, these are the essential ingredients:
Brandy and double cream. Everything else is an add on. Having said that, a really good blast of pepper is really good.
Personally, I always think a really good piece of steak needs nothing doing to it. However, a really good piece of steak can be hard to come by sometimes. In that eventuality, then this is a sauce which can make quite a difference!
Just as an aside, switch the brandy for calvados, the steak for pork and some of the onion with apple and you have one of my ultimate favorate things! But that’s for another day.
Fry off the steak to you preferred level of doneness and add the brandy.
Flambe and reduce. If you want to add butter then this is a good time to do it. For me, butter make this too rich.
Remove the steak to a hot plate to rest. While the steak is resting add onion, garlic and mushroom to the pan and soften down. As things start to soften add in the double cream.
Don’t add in too much cream or you will wreck it. A teaspoonful at a time and keep testing until you’ve got it. When the sauce is properly cooked through and pour over the steak.
This is really good with plain boiled potatoes and those small green beans. And a good bottle of red wine.