Preheat oven to 400 degrees.
Prepare rub by combining dill, mint, garlic, olive oil, salt (a good 1/2 tsp pinch), and pepper in a small food processor (or blender). Pulse until well combined. Set aside.
Drizzle olive oil over the pork tenderloin, followed by a healthy pinch of kosher salt and freshly ground pepper. Rub to coat all over.
Heat a stove-to-oven cast iron grill on the stove-top on medium heat. Allow it a few minutes to get good and hot. Note: do not oil the grill.
Place the tenderloin on the hot grill and sear each side for two minutes until brown. Don't skip over this step! This is where so much of the flavour develops. Once all sides are nicely browned, use a spatula or grill brush to rub the dill and mint mixture all over the tenderloin, setting aside a tbsp of the rub for later.
Place the tenderloin (still on the grill) directly into the oven and let cook for 15 minutes (if your tenderloin is much smaller, check after 10 minutes).
After 15 minutes, remove the tenderloin and slice through the middle. The tenderloin will still be quite pink, but will continue to cook while it rests. Brush the remaining rub over the tenderloin. Cover the tenderloin with aluminum foil and let rest for 10 minutes. Remove foil, slice thinly and serve with your fav sides.