1. Preheat oven to 350°.
2. Melt butter in a large nonstick skillet over medium heat. Add onion, mushrooms, and garlic to pan; cook 8 minutes, stirring occasionally. Remove from heat; cool 5 minutes.
3. Combine mushroom mixture, panko, and next 8 ingredients (through egg) in a large bowl; stir well to combine. Shape turkey mixture into a 9 x 5–inch rectangle on a broiler pan coated with cooking spray.
4. Combine ketchup, brown sugar, mustard, and nutmeg in a small bowl, stirring with a whisk. Spread ketchup mixture evenly over top of meat loaf; bake at 350° for 40 minutes or until a thermometer registers 160°. Let stand for 10 minutes before serving.