Season the pork tenderloin with salt, pepper and coriander seeds power and let it rest to bring the meat to room temperature.
Preheat oven to 350°. Place a ovenproof large skillet over high heat. When the pan is hot add the oil. When the oil is hot, lay the pork tenderloin in it. Let it cook without moving if for a 2 minutes or until nicely browned.
Add the butter, garlic and rosemary to the pan. Turn the tenderloin.
When the butter has melted, spoon it all over the tenderloin and place in the hot oven.
After 5 minutes of cooking, remove the pan from the oven and baste the tenderloin. The meat should feel springy at the touch.
Return it to the oven for another 5 – 7 minutes. Baste the tenderloin again and let rest for a couple of minutes.
Cut the tenderloin crosswise slices and drizzle some of the rosemary butter sauce over it.