Make the meatloaf: Preheat the oven to 375 degrees F. Coat a 9-by-5-inch loaf pan with cooking spray. Put the turkey, onion, breadcrumbs, celery, egg, 2 tablespoons ketchup, the parsley, 1/2 teaspoon salt, and pepper to taste in a bowl. Mix with your hands until just combined, then transfer to the prepared pan. Combine the remaining 1/4 cup plus 2 tablespoons ketchup with the soy sauce and Worcestershire sauce in a small bowl; spread 2 tablespoons of the ketchup mixture over the meatloaf. Bake until the top begins to brown, about 30 minutes, then spread the remaining ketchup mixture over the meatloaf and continue baking until browned and a thermometer inserted into the center registers 165 degrees F, 15 to 20 more minutes. Let stand 5 minutes before slicing.
Meanwhile, make the mashed potatoes: Put the potatoes in a large saucepan and cover with cold water; season with salt. Bring to a simmer and cook over medium-low heat until tender, about 30 minutes. Drain, then return to the saucepan and mash with the butter. Add the milk and season with salt and pepper; continue mashing until fluffy. Put the peas in a microwave-safe bowl, sprinkle with water and season with salt and pepper. Cover and microwave until warmed through, about 4 minutes. Serve the meatloaf with the peas and mashed potatoes. Photograph by Paul Sirisalee