Feeding a crowd this season? This recipe’s a cinch to double—simply brown the pork in batches, then proceed as directed. If you can’t find instant polenta, regular is fine; start cooking it before you prep the pork so it’s ready on time.
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.