Bring Pork Tenderloin (2 pound) to room temperature. Preheat oven to 430 degrees F (220 degrees C).
In a mortar and pestle, crush the Black Peppercorns (2 teaspoon), Dried Rosemary (1 teaspoon), Fennel Seeds (1/2 teaspoon), Celery Seeds (1/2 teaspoon), and Kosher Salt (1/2 teaspoon). If a mortar and pestle is not available, use a spice grinder and mix in a bowl after each ingredient is ground and measured.
Coat the whole pork in the herb rub on both sides.
In a cast iron skillet, heat the Sunflower Oil (2 tablespoon). Once hot, add the herb-crusted tenderloin to sear.
Once it forms a nice brown crust, flip the pork over and finish searing all sides.
Roast the pork in the oven for 5 to 7 minutes depending on your preferred doneness.
Let the pork rest for 5 minutes.
Slice the pork into 1/2 inch slices.
Serve with some cranberry mostarda on top and enjoy!