Preheat oven to 400° and grease bottom of a roasting pan.
Take out enough mustard from the jar and place it in a little bowl. (This will insure that you won't have to touch the main jar with dirty hands after touching raw pork.)
In a small bowl, combine brown sugar, garlic powder, onion powder, salt, and pepper. Mix well.
Take pork tenderloin out of the package and place it in the prepared roasting pan.
Rub pork with Dijon mustard all over, even on the bottom.
Sprinkle the seasoning all over the top of the pork and sides. Pat the seasoning down.
Place roasting pan in the oven and cook for 18-25 minutes. Time will depend on the size of the meat. Have a thermometer ready and start checking internal temperature after 18 minutes.
For juicy pork, it’s important not to overcook it. Cook pork until the internal temperature of the thickest side reads 145º.
Take it out and let it rest for about 5 minutes before cutting into it.