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.
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 5inch rectangle on a broiler pan coated with cooking spray.
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 16