Season the beef all over with salt and pepper. As a guideline, I use about 2 teaspoons of salt total.
Heat a large skillet over medium high heat and add the fat. Sear the beef on all sides, until deeply brown. This should take about 5 minutes.
Remove the beef to a sheet pan, then roast in the oven for 20-25* minutes, for a rare roast, until the meat measures 120F with an instant read thermometer (the temperature will rise about 5 degrees as it rests).
In the meantime, melt the butter in the pan you cooked the beef in, and add the mushrooms. Season with a pinch of salt, about ⅛ teaspoon, and cook for about 10 minutes, until all of the moisture released from the mushrooms has cooked off.
Add the garlic, onion, rosemary, thyme, and beef stock. Bring the sauce to a boil, and cook for about 8-10 minutes until the liquid reduces by about half.
Remove the garlic, onion, rosemary, and thyme from the pan. The sauce is now ready to stand by for the beef, which should be just about done by now.
When the beef has finished cooking, let rest for 10 minutes. Slice the beef and serve with the mushroom pan sauce. Enjoy!
*Cooking time will vary slightly depending on the thickness of the beef tenderloin you use. Make sure to check for desired doneness with a thermometer.