Trim and tie the beef tenderloin to attempt to make it as uniformly sized as possible. Rub the roast with salt and pepper. Set aside for 30 minutes.
Meanwhile, mash together the softened butter, garlic, mustard, rosemary and thyme. Set aside at room temperature until needed.
Preheat the oven to 425°F. Heat a large (12-inch or larger) oven-safe cast iron or stainless steel skillet over medium-high heat. If using stainless steel, use canola oil to prevent sticking. This isn't necessary using seasoned cast iron.
Brown all sides of the roast, turning on each side for 1-2 minutes. Remove from the heat.
Surround the roast with sliced onion. Rubbed the fresh thyme and rosemary between your palms and add surround the roast.
Please into the oven and roast for 5 minutes. Then reduce the heat to 350°F. Top the roast with 2 tablespoons of compound butter. Roast for an additional 10 minutes, top with the remaining butter and roast for another 5 minutes. The exact time will depend on the desired degree of doneness and size of your roast. Test using a digital thermometer and remove 5°F before the desired temperature.
Tent loosely with aluminum foil and rest for 15 minutes.
Slice into 1 inch thick slices. Serve with the roasted onion slices and discard aromatics.
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