Add one tablespoon of butter to a large skillet or saute pan set over medium heat. Once the butter has melted add the sliced mushrooms and saute until tender. Once tender, remove mushrooms and set them aside.
Reduce the heat to medium low and melt the remaining butter in the pan, then sprinkle in the flour. Whisk while cooking for three minutes, to prevent the flour from scorching.
Slowly begin to dribble in beef broth and bourbon in batches. Whisk to incorporate the liquid into the flour mixture prior to adding more. Repeat until all broth, bourbon, and heavy cream has been added.
Add the mushrooms back into the pan along with the sprig of thyme, salt, and pepper to the sauce and gently simmer for a couple of minutes to thicken.*
Once thickened, remove from heat and serve over freshly grilled steak.
The longer it simmers, the thicker the gravy will become. Add a little more liquid if it cooks too long and needs to be thinned out.