1. Heat the oven to 400°. Season the tenderloins with 2 teaspoons of the coriander, 1/2 teaspoon of the salt, and the pepper.
2. In a large ovenproof frying pan, heat the oil over moderately high heat. Add the tenderloins and brown on all sides, about 3 minutes. Put the pan in the oven and cook until the tenderloins are just done to medium, about 15 minutes. Transfer the tenderloins to a carving board and leave to rest in a warm spot for about 5 minutes.
3. Pour off all the fat from the pan. Put the pan over moderate heat and add the port. Bring to a boil, scraping the bottom of the pan to dislodge any brown bits. Add the broth, prunes, and the remaining 1/4 teaspoon salt. Boil until reduced to approximately 1/2 cup, about 3 minutes. Whisk in the remaining 1/2 teaspoon coriander and the butter. Slice the tenderloins and serve topped with the prunes and sauce.
Wine Recommendation: A light-bodied or subtly flavored red wine would fade away next to the intense fruity flavors of this dish. Fortunately, lusty red zinfandels are widely available and will work perfectly.