Step 1: In a large mixing bowl, gently combine the ground beef, ground chicken, breadcrumbs, egg, milk, and finely chopped onion. Season with salt and pepper. Be careful not to overmix, as this can lead to tough meatballs.
Step 2: Form the meat mixture into uniform balls, about 1 to 1½ inches in diameter. Using a cookie scoop can help ensure consistent sizing for even cooking. Place the formed meatballs on a parchment-lined tray.
Step 3: (Optional, but recommended for extra flavor) In an oven-safe skillet, brown the meatballs lightly over medium-high heat for about 2 minutes per side. This helps develop a delicious crust and locks in moisture. Transfer the browned meatballs directly into your crock pot. If not browning, place raw meatballs directly into the crock pot.
Step 4: Prepare the cream sauce. In a medium saucepan, melt the butter over medium heat. Whisk in the all-purpose flour to create a roux, cooking for 1–2 minutes until lightly golden.
Step 5: Gradually whisk in the beef broth, stirring constantly to prevent lumps. Continue to cook, stirring, until the sauce thickens.
Step 6: Remove the saucepan from the heat. Stir in the heavy cream (or sour cream – if using sour cream, allow it to come to room temperature and whisk in gently to prevent curdling), Worcestershire sauce, allspice, and nutmeg. Season with salt and pepper to taste.
Step 7: Pour the creamy sauce over the meatballs in the slow cooker, ensuring they are evenly coated.
Step 8: Cover the crock pot and cook on the low setting for 4–5 hours, or on the high setting for 2–3 hours.
Step 9: About halfway through the cooking time, gently stir the sauce to ensure even consistency and prevent any sticking to the bottom.
Step 10: Before serving, use a digital meat thermometer to confirm that each meatball has reached a safe internal temperature of 160°F (71°C).
Step 11: Once cooked, you can switch the crock pot to the “warm” setting for up to 30 minutes until you’re ready to serve.
Step 12: Serve hot over your favorite accompaniment, such as egg noodles, mashed potatoes, or rice, and garnish with fresh chopped parsley or dill for a burst of freshness.