In small bowl, combine Dijon and balsamic (please note that this is balsamic vinegar and NOT vinaigrette).
On a cutting board, lay out tenderloins. Generously slather with half of dijon mixture. Sprinkle each with 1 tbsp of rosemary.
Add oil to cast iron and rub around to evenly coat. Put the tenderloin in the pan, dijon mixture side down. Slather with remaining dijon mixture. Sprinkle each with 1 tbsp of rosemary.
Place in oven for 45 minutes. Let stand at least 5 minutes before slicing and serving.