Season the steak with salt and pepper on both sides.
Heat the olive oil and butter in a pan over medium high heat.
Add steaks and let it sear until brown (3-4) minutes. Turn it and sear the other side.
Remove steaks from pan and set aside.
To the same pan, add garlic, mushrooms and thyme. Cook for 2-5 minutes until the mushroom juices have mostly evaporated.
Add whiskey and stir. Let the alcohol evaporate.
Add heavy cream and mustard. Cook it until it is reduced by 1/3. Add steaks back into sauce and let it reduce further while cooking the steaks.
Once the steak is cooked to your liking, remove it from the pan and let it rest.
If the sauce it is still runny, let it reduce it a while longer. You should end up with a thick sauce that is 1/3 or less of what you started with.
Remove the sprig of thyme. Pour the sauce on top of steaks and serve.