Preheat your oven to 375°F and line a large baking sheet with parchment paper.
Make the meatballs: In a large mixing bowl, combine the ground chicken, grated onion, eggs, breadcrumbs, nutmeg, allspice, salt, and pepper. Mix until well combined.
Form the meatballs: Use a small cookie scoop (or your hands) to make evenly sized meatballs. Place them on the prepared baking sheet. A 1.5 tablespoon cookie scoop will make about 35 meatballs.
Bake: Bake for 20 minutes to partially cook the meatballs.
Make the gravy: In a large, heavy-duty skillet or Dutch oven, melt the butter over medium-low heat. Once melted, whisk in the flour to form a smooth paste. Cook the paste for about 30 seconds, then slowly stir in the beef broth and cook, whisking, until the mixture has thickened into a gravy.
Temper the sour cream: Scoop a few tablespoons of hot gravy into a small bowl with the sour cream. Stir until smooth, then whisk it back into the gravy.
Finish the gravy: Stir in the heavy cream, Dijon mustard, and Worcestershire sauce. Taste and season with salt and pepper as needed.
Simmer the meatballs: Transfer the partially baked meatballs into the gravy, gently stirring to coat. Cover and simmer over low heat for about 10 minutes, or until the meatballs are cooked through (internal temperature of 165°F). Crack the lid slightly to prevent the sauce from getting too hot and burning to the bottom of the pan.
Serve over egg noodles or mashed potatoes and enjoy!