In a mortar and pestle, combine coriander, fennel and red pepper flakes. Use the pestle to crack and grind the spices in the mortar, until the spices are crushed and fragrant.
In a small bowl, add the rosemary, garlic, salt, pepper and crushed spices. Stir to combine.
Place the pork on a baking sheet. With your hands, rub the olive oil over the pork. Sprinkle spice mixture over the pork and rub it in with your hands. There should be enough spice to liberally cover the entire tenderloin.
Cover loosely with plastic wrap and refrigerate at least one hour or up to 8 hours.
Heat your grill to a medium high heat (about 425-450 degrees). Grill pork, turning occasionally, 10-12 minutes for medium-rare to medium or until desired doneness is achieved.
To roast pork in an oven, preheat oven to 425 degrees. Place pork on a baking sheet and roast for 18-20 minutes, until desired doneness is achieved.