First I have the butcher trim and tie it so I can roast it whole. To one jar of Dijon mustard I add a LOT of finely chopped garlic and 2 heaping tablespoons of brown sugar. Place the tenderloin on a rack in an open roasting pan and cover liberally with the mustard mixture. Preheat the over to 475 or 500 degrees, convection oven if you have it. Roast till an instant read thermometer reads 115 (for rare). Start checking the temperature after 10 or 15 minutes. Time will vary significantly on the thickness of the tenderloin and whether you have convection or not. Remove from the over and let rest for 5 -10 minutes before slicing. you can heat and serve extra mustard mixture as a sauce.