Preheat oven to 450 degrees. Lightly oil a 9-by-13-inch roasting pan; set aside. In a small bowl, mix oregano, coriander, 1 1/2 teaspoons salt, and 1/2 teaspoon pepper. Rub mixture evenly over pork tenderloins. Place pork in prepared pan.
In a small saucepan, bring vinegar and brown sugar to a boil over high heat; cook, stirring occasionally, until mixture is reduced to 1/2 cup, about 3 minutes. Remove from heat.
Roast pork 5 minutes; brush with 1/3 of balsamic mixture, coating entirely. Roast another 5 minutes; brush with remaining balsamic mixture. Continue roasting until internal temperature registers 155 degrees on an instant-read thermometer, about 10 minutes more. Remove from pan. Let rest 10 minutes; slice 1/4 inch thick.
Spread mustard over split rolls, then layer half with pork and onions. Top with remaining bread halves.
You can roast the pork tenderloin up to a day ahead of time; let cool, then cover and refrigerate. Slice the pork just before you are ready to assemble the sandwiches. Red onions are sweeter than most yellow onions, but they can still be strong. For a milder taste, soak the onion slices in cold water for about ten minutes, then drain and pat dry before placing them on the sandwiches.