Preheat oven to 450 degrees. Lightly oil a 9-by-13-inch roasting pan. On a sheet of parchment paper, combine salt, oregano, coriander, and pepper. Working with one tenderloin at a time, roll to coat with spices. Transfer to prepared pan.
In a small saucepan, bring vinegar and sugar to a boil. Cook, stirring occasionally, until mixture is syrupy and reduced to 1/2 cup, about 3 minutes. Remove from heat.
Roast the pork for 5 minutes. Brush all over with 1/3 of the balsamic reduction. Roast 5 minutes more, and brush with remaining balsamic mixture. Roast until the temperature registers 155 degrees on an instant-read thermometer, about 10 minutes more. Remove from pan. Let rest 10 minutes before slicing.
Split the rolls, and brush 1/2 teaspoon of the mustard over each side. Place meat on rolls and top with onion. Replace the tops, and serve immediately.