In a large bowl, combine the ground beef, ground pork, breadcrumbs, egg, milk, minced onion, nutmeg, salt, and pepper. Mix gently until just combined — avoid overmixing — then let the mixture rest for 5 minutes so the breadcrumbs absorb the moisture.
Using a cookie scoop or your hands, portion the meat mixture into balls about 1.5 inches in diameter and roll each one smooth between your palms. You should get approximately 24 to 30 uniform meatballs.
Heat a large skillet over medium-high heat with a small amount of oil and brown the meatballs in batches on multiple sides, about 10 minutes total — you're building a crust, not cooking them through. Transfer each batch to the slow cooker as you finish.
Whisk together the beef broth and flour in a bowl until completely smooth with no lumps, then add the Worcestershire sauce, Dijon mustard, and garlic powder and whisk to combine. Pour the sauce over the browned meatballs in the slow cooker.
Cover and cook on high for 3 to 4 hours or on low for 5 to 6 hours, until the meatballs reach an internal temperature of 165°F and the sauce is bubbling. Avoid lifting the lid during cooking.
About 20 minutes before serving, switch the slow cooker to low, ladle out about 1 cup of hot cooking liquid, and whisk it together with the sour cream in a separate bowl to temper it. Stir the mixture back into the crockpot and heat through for 15 to 20 minutes.
Taste and adjust seasoning with salt and pepper, then garnish with freshly chopped parsley. Serve over egg noodles, mashed potatoes, or rice with plenty of sauce.