Place panko and milk in a large bowl; stir until well combined. Let stand 5 minutes.
Add turkey, onions, eggs, 1/4 cup ketchup, parsley, mustard, salt, sage, and pepper to breadcrumb mixture. Gently mix until well combined. Transfer mixture to a 9 3/4-inch-by-5 3/4-inch loaf pan. Brush top with remaining 2 tablespoons ketchup.
Transfer meatloaf to oven and bake, draining any accumulated juices as necessary, until it reaches 165 degrees on an instant-read thermometer, 1 1/2 to 1 3/4 hours. Serve immediately or use to prepare sandwiches.