Step 1

Sauce and meatballs, part 1: Start the sauce by adding beef stock to a saucepan over medium-high heat; boil, uncovered, until reduced by half, 15 minutes. Meanwhile, prepare the meatballs: In a medium bowl, whisk together bread crumbs, milk, allspice, salt, and mustard. Add the egg and whisk again. Allow liquid to absorb into the breadcrumbs, 10 minutes.

Step 2

Meatballs, part 2: Line a rimmed baking sheet with parchment paper. Place beef and pork in a large bowl (or just beef, if you aren't using pork); add the milk and breadcrumb mixture, and combine well with your hands. Roll into balls, about 1 ounce (2 tablespoons) each, and set aside on the baking sheet. (Use a cookie scoop to measure out the balls, if you wish.) Chill 45 minutes to one hour.

Step 3

Sauce, part 2: Add cream to the reduced beef stock. Leave over medium heat, uncovered, until reduced by ⅓, about 30 minutes. (Stay nearby to make sure the sauce doesn't boil over.)

Step 4

Potato purée: Add potatoes to a medium saucepan with a big pinch of salt and enough cold water to cover. Bring to a simmer over medium-low heat and cook until very soft and almost falling apart, 20 minutes. When potatoes are cooked, drain well; then place them back into the hot pot (with the heat turned off) to evaporate any residual moisture, 30 seconds. Run potatoes through a food mill or ricer into a mixing bowl. Add butter and stir well to combine; then pour in the warm milk and whisk until thoroughly combined. Add salt to taste and place back into the the saucepan. Over very low heat, whisk again to release extra moisture in the potatoes. Set aside, uncovered, while making meatballs.(Note: Potato purée can be made a day ahead and stored in a covered container in the refrigerator. To reheat, add to pot with a splash of milk and stir over low heat until hot.)

Step 5

Sauce, part 3: When the cream has reduced by ⅓, finish the sauce by stirring in the lingonberry jam. Add salt to taste. (Chef Bengtsson suggests adding salt to all sauces after they have reduced.)

Step 6

Meatballs, part 3: Melt 3 tablespoons butter in a large nonstick skillet over high heat. When butter is foamy, add first batch of chilled meatballs in an even layer. Reduce heat slightly to medium-high and cook on all sides until browned on the outside but still slightly pink in the middle, about 5 minutes. Cook remaining meatballs in 1–2 more batches, adding more butter if necessary. If butter browns too fast or starts to burn, wipe out the pan and use fresh butter. (Meatballs can be made in advance up to this point and refrigerated for 5-6 days or frozen in zip-top bags for up to 2 weeks.)

Step 7

Assembly: Strain cream sauce into a bowl through a fine-mesh strainer to ensure smooth consistency. Place a big spoonful of potato purée on a plate, along with 6–8 meatballs and a dollop of lingonberry jam. Spoon cream sauce over the meatballs and garnish with chopped parsley. Serve immediately.