Preheat oven to 450 degrees F, then put the pork tenderloin in a roasting pan.
In a small bowl, combine 2 tablespoons of the brown sugar with the Dijon mustard, paprika & ginger, then spread HALF of this mixture over the pork tenderloin, before baking uncovered for 15 minutes.
When the 15 minutes are up, spread the pork with the remaining brown sugar mixture & bake 15 to 20 minutes longer, or until a meat thermometer reads 145 degrees F, but if you're a traditionalist, you'll want the thermometer to read 160 degrees F!
Meanwhile, for the salsa, in a small mixing bowl, combine the bell pepper & onion with the last tablespoon of brown sugar, along with the cup of diced pineapple & pineapple juice.
When the pork comes out of the oven, let it stand briefly before slicing the meat & serving it with a generous portion of salsa.