I am using porcini mushrooms for this recipe, but you can use any flavorful mushroom. Other than regular button mushrooms, some good options are shiitake, beech mushrooms, hen of the woods, black trumpets, other boletes, morels and even chanterelles.
The best way to make this sauce is something of a two-step: Sear the mushrooms first, then cook the steak in the same pan, then, while the steak is resting, finish the sauce. So start with a large saute pan set over high heat. Add all the mushrooms. Shake the pan and let them release their water, which should happen in a minute or three.
Add the butter and toss to combine. Saute the mushrooms until nicely browned, then remove them to a bowl with any stray butter. Wipe out the pan and cook the steaks.
When the steaks are resting, add the minced shallot to the pan, adding a little butter if it's too dry. Brown the shallot for a minute or two -- it will pick up any browned bits from the pan quickly -- and add the mushrooms and any juices from the bowl. Toss to combine.
Add salt, pepper and thyme, then the tomato paste. Mix well and let this cook a minute. When the tomato paste is well incorporated, add the venison or beef stock and mix well. Pour in the brandy and bring everything to a boil. Let this boil until it thickens to the consistency of heavy cream, which will take a couple minutes depending on how big your pan is.
Turn off the heat and stir in the heavy cream and the minced fresh herbs. Taste once more for salt and serve.