Make the rub: combine the garlic powder, tablespoon of dried thyme, salt and sage in a small bowl.
Preheat the oven to 400 on the convection setting, if you have that option, or 425 conventional if you do not.
Lay the pork tenderloin out on a work surface, and spread the rub over the pork. Rub it into the meat, and let sit for about 15 minutes at room temperature.
Place the pork into a roasting pan, and put in the preheated oven.
Make the mushroom sauce as the pork roasts, which takes about 20 minutes. Heat the olive oil in a skillet on medium heat then add the onions. Cook for about 5 minutes until they begin to soften.
Add the mushrooms, and cook for 5 minutes or so, until the mushrooms begin to sweat. Add the red wine and the 1/2 tsp of thyme. Simmer until the wine is mostly absorbed, about 5 more minutes.
Stir in the cream to finish the mushroom sauce, and add salt and pepper to taste. When the cream starts to bubble, lower the sauce pan to very low, to keep warm for the pork.
The pork should take about 20 minutes to roast to doneness, though you should check it a few minutes before hand to avoid over cooking. Pork tenderloin should still have a little pink showing, registering between 140 and 145 degrees when you remove it from the oven. Set aside to rest for about 5 minutes.
Slice the pork, and serve topped with the mushroom cream sauce. Enjoy!