Heat oven to 350. Place popcorn in a 9x13-inch baking pan that has been lined with foil; set aside. Place butter in small bowl and microwave about 20 seconds or until melted.
Pour butter over popcorn and toss to coat. Sprinkle Parmesan cheese and lemon pepper over popcorn and toss again. Bake 7-10 minutes or until heated through. Toss again just before serving. Serve immediately or cool completely and store in an airtight container.