Slow roasting a beef tenderloin is the best method for insuring a perfectly cooked interior with a flavorfully seared exterior.
Preheat oven to 250 degrees. Line a half sheet pan with foil. Place a metal baking rack on top of the foil.
Rub the garlic salt and sea salt evenly over all sides of the beef tenderloin roast. Allow to rest on the counter - 60 minutes.
Slow roast the tenderloin until the center of the roast reads 135 degrees F on a digital thermometer. Mine took about 90 minutes. Start checking your temperature after 70 minutes to be sure not to over-roast it.
Heat the butter in a large (12-inch) skillet over medium high heat until melted and bubbling. Using tongs transfer the beef roast to the skillet and brown on all sides, just about one minute per side. Transfer the roast to a serving platter.
Add the wine to the same skillet and allow it to simmer vigorously for 5 minutes. Whisk in the mustard and add the pepper.
Spoon the sauce over the roast and serve with simple roasted potatoes and asparagus.