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1
Cover potatoes with water in a large saucepan and place over medium-high heat. Bring to a boil and simmer until tender when pierced with a fork, about 20 minutes. Drain and return to pan. Add 3 tablespoons butter, Parmesan, and milk. Season with salt and pepper and whip potatoes using a hand-held mixer until airy, but do not over-mix.
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2
Heat olive oil in a medium skillet over medium heat until shimmering. Add onion, celery, and garlic and cook, stirring often, until tender and translucent, about 7 minutes. Transfer to a large bowl and let cool.
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3
Adjust oven rack to lower-middle position and preheat oven to 375u00b0F. Whisk eggs and chicken stock in a medium bowl. Sprinkle with gelatin and let stand for 5 minutes.
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4
Add the beef, pork, veal, Worcestershire sauce, mustard, tomato paste, paprika, thyme, 3/4 teaspoon salt, 1/2 teaspoon black pepper, saltines, and Parmesan cheese to bowl with cooled onion mixture. Add egg mixture and combine using hands. Do not over-mix.
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5
Spray loaf pan with cooking spray. Pack half of the meat lightly and evenly into the loaf pan, creating a well lengthwise down the center and leaving about an inch not indented on either end. Fill well with peas and carrots. Top with remaining meat, pressing gently to fuse top and bottom halves. Evenly spread mashed potatoes on the top and make a crosshatch pattern using the tines of a fork. Cut remaining 2 tablespoons of butter into small pieces and dot them on top of the potatoes. Transfer meatloaf to the oven to cook.
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6
While meatloaf cooks, combine cheese, cream, and pepper in a small saucepan. Heat over medium-low heat, stirring frequently, until cheese is melted and sauces has thickened, about 10 minutes. Cover and keep warm.
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7
The meatloaf is done cooking when the potatoes are browned and an instant-read thermometer inserted into center of loaf reads 135u00b0F to 140u00b0F, 55 to 65 minutes.Remove from oven and let rest for 15 minutes before slicing. Serve immediately, passing Stilton sauce at table.
