When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 325 degrees F and preheat, lid closed, for 10 to 15 minutes.
Transfer chicken livers to a cup that just fits head of immersion blender and purée until smooth.
Heat olive oil in a large Dutch oven over high heat until shimmering. Add ground beef, pork, and lamb. Season with salt and pepper and cook, stirring to break up meat with a wooden spoon. Cook until browned. Remove from heat and stir in puréed chicken livers.
Heat butter and pancetta in a large skillet over medium-high heat and cook, stirring frequently, until fat is rendered but butter and pancetta have not browned, about 8 minutes.
Add onion, carrots, celery, garlic, sage, and half of parsley and cook, stirring until vegetables are completely softened but not browned, about 8 minutes. Transfer mixture to Dutch oven with meat mixture.
Return Dutch oven to high heat. Cook, stirring, until most of the liquid has evaporated from the pan, about 10 minutes longer.
Add the wine and cook, stirring until most wine has been evaporated. Add reserved stock, tomatoes, milk, and bay leaves. Season with salt and pepper.
Bring sauce to a simmer, then transfer to Traeger. Cook uncovered, stirring every 30 minutes, until liquid has almost completely reduced and sauce is rich and thick, about 3 hours.
Skim off most of the fat, leaving behind about 1 cup.
Stir in heavy cream, parmesan, and remaining parsley. Bring to a boil on stovetop, stirring constantly to emulsify. Season to taste with salt and pepper. Keep warm.
Cook pasta by package directions in a large pot of salted water until al dente. Drain and reserving a 1/2 cup of water. Return pasta to pot and add sauce to just coat the pasta as well as the 1/2 cup of reserved cooking liquid.
Place pot over high heat on stove, tossing and stirring gently, until sauce is thick and pasta is coated, about 30 seconds.
Transfer to a serving bowl and serve immediately. Garnish with fresh grated parmesan and minced flat leaf parsley. Enjoy!
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