Preheat a large skillet over high heat. Pour in the oil, and heat until smoking. Sear the tenderloin roast in the hot oil until browned on all sides, about 5 minutes. Remove to the roasting pan when done. While the roast is searing, place the garlic, mustard, avocado, and steak seasoning into a blender or food processor. Blend until smooth. Coat the seared roast all over with the avocado mixture.
Roast in preheated oven until the tenderloin has reached your desired degree of doneness, about 40 minutes for medium.