Whisk Savory Gravy Base: In the insert of a 5 to 6-quart slow cooker, whisk together the beef broth, condensed cream of mushroom soup, Worcestershire sauce, Dijon mustard, garlic powder, onion powder, ground nutmeg, ground allspice, kosher salt, and freshly cracked black pepper until smooth and fully blended.

Add Meatballs & Slow Cook: Add the homestyle meatballs directly into the slow cooker. Gently fold with a silicone spatula until every meatball is thoroughly coated in the gravy base. Cover with the lid and cook on LOW for 3 to 4 hours (or on HIGH for 1.5 to 2 hours) until meatballs are piping hot (internal temp 165°F).

Thicken the Sauce: 20 minutes before serving, whisk the cornstarch and cold water together in a ramekin until smooth. Pour the slurry into the slow cooker and stir gently. Cover and cook on HIGH for 15 minutes until the gravy noticeably thickens into a glossy glaze.

Temper & Add Cream: Switch the slow cooker setting to WARM. In a heatproof bowl, whisk together the heavy cream and room-temperature sour cream. Ladle 1/2 cup of the hot gravy from the slow cooker into the cream mixture and whisk continuously to temper (this prevents any curdling). Pour the tempered cream back into the crock and gently fold into the meatballs until a smooth, velvety light tan Swedish gravy forms. Cover for 5 minutes on WARM.

Garnish & Serve: Sprinkle generously with finely minced fresh parsley. Serve warm directly from the crockpot with wooden cocktail picks for an appetizer, or ladle over warm buttered egg noodles accompanied by a spoonful of tart lingonberry jam.