Start by slicing your package of mushrooms. Remove the stems so that they sit evenly on the chopping board. Now, thinly slice all the mushrooms.
They don't need to be perfect, you just want a similar thickness so that they all cook at about the same time and rate. They'll add a really nice flavor and texture to the sauce.
To chop the garlic, crush it first. This helps the skin come off easily. Take the skin off and chop nicely, then mince. Now, mince your shallots. Shallots add that special onion element without being too harsh or too overpowering.
Pick your fresh thyme leaves and give it a quick chop to make sure that the flavor is easily dispersed throughout the sauce. Get your butter ready.
Take out your steak and brush a little oil on it. Season the steak with kosher salt, and fresh cracked pepper. Don't forget to generously season the other side.
Preheat the skillet to medium high. Put the steak directly in the pan and allow to get a hard sear on the first side. You don't need to use any oil since the steak has been nicely coated.
Once they've seared for a couple minutes, flip to the other side and allow to sear too for about another two or three minutes to really develop that crust.
Get them out and place them in a 365 degree heated oven while we make our sauce. To make the sauce, use the same pan. Add some butter and let it melt. Then go in with your mushrooms.
It is important to cook out a good number of the mushroom moisture before adding the other ingredients. Now, add some more butter. Now, add your shallots and garlic to the mix.
Now, add your unsalted beef stock and heavy cream. Here's a trick to know when your sauce is ready, it should be thick enough to coat the back of a spoon and keep a line when you run your finger through.
Add your thyme now. It brings a little freshness to the sauce. Get the steak out of the oven and serve it on a bed of sauce.