Remove roast from refrigerator at least 15-20 minutes before roasting.
Place oven rack in center position and pre-heat oven to 350°F.
Heat 1 tablespoon olive oil in a large, high-sided skillet over medium heat. Add the mushroom and shallots and season with salt and black pepper, to taste. Cook, stirring occasionally, until the mushrooms and shallots soften, approximately 4-5 minutes. Remove from heat and transfer to a bowl and set aside.
In a small bowl, combine olive oil, rosemary and garlic and season with salt and black pepper, to taste. Rub the rosemary-garlic mixture on top and all along the sides of the beef.
Arrange the sliced red onion on the bottom of the skillet then place the seasoned beef tenderloin on top of the onions.
Arrange the mushroom and shallot mixture around the sides of the beef and place in the pre-heated oven until internal temperature with an instant read thermometer reaches 140°F for medium-rare, approximately 45-50 minutes.
Remove from oven and rest for 10-15 minutes before serving. Enjoy!