Preheat oven to 400 degrees. Pat pork dry, rub with 2 tsp oil. Chop rosemary. Stir together rosemary, .5 tsp salt, and .5 tsp pepper in a small bowl and sprinkle over pork.
Heat an ovenproof skillet over medium-high. Add pork and cook until browned on all sides, about 4 minutes. Transfer skillet to oven and roast until the temperature in the thickets portion of the tenderloin is 145 degrees, about 14-16 minutes. Transfer pork to a cutting board, saving the drippings from the skillet. Let the pork rest 10 minutes.
While pork rests, add remaining 4 tsp oil to drippings in skillet and heat over medium-high heat. Add mushrooms, .5 tsp salt, and .5 tsp pepper and cook stirring often until mushrooms begin to brown, 4-5 minutes. Add leeks and cook until mushrooms are deeply browns and leeks are tender, about 6 minutes. Stir in parsley and butter until melted.
Cut pork into 12 slices and serve with mushrooms on top.