1 pound Pork Tenderloin
2 tablespoons Canola Oil, divided
1 tablespoon chopped Fresh Rosemary
1 1/4 teaspoons kosher Salt, divided
1 teaspoon Black Pepper, divided
12 ounces mixed Fresh Mushrooms, stemmed and coarsely chopped (about 3 cups)
1 cup sliced Leeks
2 tablespoons chopped fresh flat-leaf Parsley
2 tablespoons Unsalted Butter, divided
3/4 cup uncooked Instant Polenta
2 tablespoons plus 2 tsp. shredded Parmesan Cheese
Preheat oven to 400°F. Pat pork dry; rub with 2 teaspoons oil. Stir together rosemary, 1/2 teaspoon salt, and 1/2 teaspoon pepper in a small bowl; sprinkle over pork. Heat an ovenproof skillet over medium-high. Add pork; cook until browned on all sides, about 4 minutes. Transfer skillet to oven; roast at 400°F until a thermometer inserted in thickest portion reaches 145°F, 14 to 16 minutes. Transfer pork to a cutting board, reserving drippings in skillet. Let pork rest 10 minutes.
While pork rests, add remaining 4 teaspoons oil to drippings in skillet; heat over medium-high. Add mushrooms, 1/2 teaspoon salt, and remaining 1/2 teaspoon pepper; cook, stirring often, until mushrooms begin to brown, 4 to 5 minutes. Add leeks; cook, stirring often, until mushrooms are deeply browned and leeks are tender, about 6 minutes. Stir in parsley and 1 tablespoon butter until melted. Remove from heat; cover to keep warm.
Prepare polenta according to package directions, omitting salt and fat. Whisk Parmesan, remaining 1 tablespoon butter, and remaining 1/4 teaspoon salt into warm polenta.
Cut pork into 12 slices. Spoon 1/2 cup polenta onto each of 4 plates; top each with 3 pieces of pork and 1/2 cup mushroom mixture.