REMOVE the pork from the refrigerator approximately 30 minutes before cooking.

PREHEAT the oven to 375°F.

If necessary, trim the pork tenderloin, removing the tough outer skin. (This is generally not necessary on boneless cuts of pork loin, but see the tip below for details.)

Pat the tenderloin dry with a paper towel. Mix the mustard powder, paprika, onion powder, garlic powder, salt, and pepper in a small bowl. Rub the pork evenly with the spice mixture.

In a food processor or by hand, pulse or chop the walnuts until finely chopped. Coat the pork evenly with three quarters of the chopped walnuts.

Place the tenderloin in a baking pan and roast for 25 to 30 minutes, until the internal temperature reaches 145°F. Let the pork rest for 10 minutes. (important: use a meat thermometer