Preheat the oven to 375 degrees. Combine the ground beef, breadcrumbs, egg, diced onion, 1/4 cup milk, allspice, salt, and black pepper in a mixing bowl. Mix until evenly combined.
Shape the mixture into a loaf and place it in a loaf pan. Bake for 45 minutes until the internal temperature reaches 160 degrees.
While the meatloaf bakes, place the potatoes in a large pot and cover with water. Bring to a boil and cook until tender, about 15 minutes.
Drain the potatoes and return them to the pot. Add 2 tablespoons butter and 1/2 cup milk. Mash until smooth and season with a pinch of salt if needed.
Warm the lingonberry preserves with the water and remaining 2 tablespoons butter in a small saucepan over low heat. Stir until the sauce is smooth and heated through.
Slice the meatloaf and serve it with the mashed potatoes and lingonberry sauce. Garnish with fresh parsley.