Instructions:
- In a large bowl, gently combine ground beef, pork, breadcrumbs, egg, milk, minced onion, salt, pepper, allspice, and nutmeg. Do not overmix.
- Roll the mixture into small, even balls (about 1 to 1.5 inches in diameter). Place them onto a plate or baking sheet.
- Heat butter/oil in a large skillet over medium-high heat. Brown the meatballs on all sides until a nice crust forms (they do not need to be cooked through). Transfer the browned meatballs directly into the slow cooker insert.
- In the same skillet, melt the 2 tablespoons of butter over medium heat. Whisk in the flour and cook for 1 minute until smooth (this is your roux).
- Slowly whisk in the beef broth until the mixture is smooth and slightly thickened. Bring to a gentle simmer.
- Remove the mixture from the heat. Whisk in the heavy cream, sour cream, Worcestershire sauce, Dijon mustard, and vinegar. Season generously with salt and pepper.
- Pour the finished sauce evenly over the meatballs in the slow cooker.
- Cover and cook on Low for 6–8 hours or High for 3–4 hours, until the meatballs are cooked through and the sauce is velvety smooth.
- Taste the sauce before serving and adjust seasoning (add more salt, pepper, or a tiny splash of vinegar for brightness) if needed.