In a stand mixer fitted with the paddle attachment, soak the bread in the milk until softened, about 5 minutes. Mix on low speed until uniform, about 30 seconds.
Add the beef, pork, onion, and egg and mix on low speed until incorporated. Sprinkle in the allspice, nutmeg, 1/2 tsp salt and 1/2 tsp pepper. Increase the speed to medium, and mix until incorporated, about 30 seconds.
Set a small bowl of cold water nearby and, occasionally moistening your hands, roll 1-1/2 oz portions of the meat into balls; you should get about 25.
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 375 degrees F and preheat, lid closed, for 10 to 15 minutes.
Heat a 12-inch cast iron skillet on the Traeger. Add 1 Tbsp of the butter and 1 Tbsp of the oil to the skillet.
When the butter is melted, add half of the meatballs. Reduce the heat to 300 degrees F and cook, undisturbed, until browned on the bottom, 2 minutes.
Flip and brown the other side, 1 to 2 minutes more. Transfer to a paper towel lined plate, and repeat with the remaining meatballs.
Add the remaining 1 Tbsp butter and 1 Tbsp oil. Add the flour, and cook, stirring with a wooden spoon, until the mixture turns light brown, about 1 minute. Whisk in the chicken broth and heavy cream. Bring to a boil.
Return the meatballs to the pan and simmer, shaking the pan occasionally, until the meatballs cook through (internal temperature of 160 degrees F) and the sauce thickens, 8 to 10 minutes. Season sauce to taste with salt and pepper.
Cranberry Relish: In a food processor, pulse the cranberries, sugar, lemon zest and juice, ginger, and 1/4 tsp salt until finely chopped. Add more sugar and salt to taste.
Transfer the meatballs to a serving bowl. Stir the herbs into the sauce, and pour over the meatballs.
Serve with the cranberry relish on the side. Enjoy!