Make the meatballs. In a large bowl, combine the beef, pork, parsley, allspice, nutmeg, grated onion, salt, pepper, garlic, panko and eggs. Mix well, then with wet hands, form golf ball–size balls (about 2 tablespoons each). You should have about 30 to 35 meatballs.
Heat the olive oil in a large skillet or braiser over medium-high heat. Once the oil is glistening, add half of the meatballs in a single layer. Cook, undisturbed, until the meatballs are just browned on the bottom, about 3 minutes. Turn them carefully and cook to brown the opposite side, about 3 more minutes. Transfer the meatballs to a plate. Repeat with remaining meatballs. Pour any excess grease from the skillet into a heatproof vessel.
Using the same skillet, reduce the heat to medium and add the butter, stirring until melted. When the butter begins to bubble, sprinkle in the flour and cook, whisking, until fully incorporated, about 1 minute. Whisk in the beef stock, salt, pepper, lemon juice, allspice and nutmeg, and cream until fully incorporated. Bring to a simmer and cook, whisking until smooth and slightly thickened.
Add the meatballs to the sauce and turn to coat. Cover and reduce the heat to low. Simmer until the gravy has thickened and the meatballs reach an internal temperature of 165°F on an instant-read thermometer, about 10 minutes.
Serve warm over mashed potatoes or egg noodles, with lingonberry jam on the side if using.
