In a large bowl, combine ground beef, ground pork (if using), breadcrumbs, milk, chopped onion, beaten egg, allspice, nutmeg, salt, and pepper. Use your hands—or a spatula—to gently fold. (Don’t overmix; you want tender meatballs.)

Wet your palms and form 1-inch meatballs, about 20 to 24 depending on size preference. Place them on a parchment-lined tray so they don’t stick.

In a heavy skillet over medium heat, melt 1 tablespoon butter with 1 tablespoon olive oil. Add half the meatballs, leaving space between each. Cook 4 minutes per side until golden brown. Transfer to a plate; repeat with remaining meatballs, adding more butter if needed.

Lower heat to medium-low. Add the tablespoon of butter left in the pan, then whisk in flour. Cook 1 minute, stirring so it doesn’t burn—a pale blonde color is your cue.

Gradually whisk in beef stock until smooth. Simmer gently for 3 minutes to thicken.

Stir in sour cream and Dijon mustard, heating until sauce is glossy and slightly thickened. Taste and adjust salt or pepper.

Nestle them into the sauce; spoon gravy over each. Cover and simmer for 5 minutes so flavors meld and meatballs reach 160°F internally.

Spoon lingonberry jam over the top, sprinkle with parsley, and plate with mashed potatoes, buttered egg noodles, or crusty bread.